Strange Jetson noise coming from my 4.2 under acceleration

[A6xEnthusiast]

Okay so here's the deal. I've been searching the internet relentlessly to see if I could find something similar to what I am experiencing with my car. I drive a 2002 Audi A6 4.2L with 160K on the clock, I have already had to replace the transmission once in this car (I have had it for a long time), and just replaced the water pump, accessory belts, and cam-chain tensioner in last 2 years. Actually the cam chain tensioner was broken (I can't remember which side). Since then, my car seemed to be running fine until in the past year my car has been making strange noise, almost like a high pitched gyrating, chugging noise, that increases in pitch as I accelerate into higher RPMs. Basically, just think of The Jetsons, that is the closest analogy I can make to the sound coming from my engine. The strange noise happens upon start up when the car has been sitting there for a while (when it is cold), it will make an intermittent high-pitched chugging sound while it idles (when it is still cold).

Do not get it confused with the injured goose sound that is related to a PCV valve malfunction. It is also not the awful high pitched noise that comes from a secondary air pump malfunction. There is also a mildly audible vacuum leak coming from the driver side of the car when the car is warm and can really only be heard when the car is idle.

Back to the strange noise, it is most audible while the car is cold, and does not go away unless I drive the car around for a bit and get the engine heated up to its optimal operating temperature. I do not have a VCDS so I can not give you guys any readings as to how the engine is behaving, what I can tell you is that about a year ago, I had to get the car taken to an Audi specialist close to where I go to college to get new keys programmed to my (wierd story) and while he was at it, he toyed around with his VCDS at got some readings on my car and informed me that the car is running rough, and that it was MAF related. The strange noise was also occurring regularly at the time the mechanic took a look at my car as well. I still have not addressed the MAF issue yet. Probably just need to clean the sensor (mechanic said it was minor). Could it be an reoccurring issue with the cam chain tensioner? I do not recall this noise occurring when my cam chain tensioner failed in the past, I also do not recall this noise happening when my transmission failed in the past either. Any insight or clues to point me in the right direction would a huge help!!

[Grnson]

Have you checked your fan clutch? I had a similar sounding noise in my 4.2 at start up and it increased under acceleration when cold. The high-pitched noise would mostly clear up after the car was up to operating temperature.

[A6xEnthusiast]

Thank you both for your replies, and that is something I can get checked out. It would make sense, given its odd noise and that it is mainly RPM dependent. I am getting ready to "unfreeze" the Intake manifold actuators and flaps this weekend, maybe I can get the clutch fan checked out while I am at it.

[A6xEnthusiast]

UPDATE: Went for a drive today just focusing on the engine noise and how it behaves. For one, I have zero loss of power, actually the car was pulling really hard today which was nice. Secondly, I noticed that even after the car got to proper operating temperature, the noise does not go away and is most noticeable when driving at low speeds. The noise does not really fully go away either. It gets better when I do WOT pulls but it does not actually really go away and is still most noticeable when driving at low speeds. Especially in first gear. It is also VERY noticeable when decelerating in first gear even after the car is more than adequately warmed up. Kind of sounds like something is either rubbing or isn't getting properly lubricated. I still cannot fully pinpoint what the sound really sounds like. It is very difficult to explain.

[A6xEnthusiast]

Another Update: the noise is not coming from the viscous fan. It is coming from the back of the engine bay up against the firewall. Honestly think it is my transmission, maybe just need to cut the bs and swap to a 6MT.

[JMURiz]

Question: does it change/go away when you turn off the a/c? Mine makes more of a noise when the A/C is on (climate control on auto instead of econ).

Experts, could it also be the SAI pump?

[brokenwrench]

And you're certain it's not a vacuum leak somewhere? Does it change when you step on the brake? It is certainly possible that the problem is caused by the TC or trans, but someone more familiar with the 4.2 tranny would have to weigh in for sure.

FWIW, the SAI pump should only come on for a short time while the engine is warming up, but since my A6 doesn't have SAI anymore I can't say for absolute certainty.

[A6xEnthusiast]

Question: does it change/go away when you turn off the a/c? Mine makes more of a noise when the A/C is on (climate control on auto instead of econ).

Experts, could it also be the SAI pump?

It does not change in regards with a/c being on or off, and it is not SAI, it has a much different noise than SAI related issues.

[A6xEnthusiast]

And you're certain it's not a vacuum leak somewhere? Does it change when you step on the brake? It is certainly possible that the problem is caused by the TC or trans, but someone more familiar with the 4.2 tranny would have to weigh in for sure.

FWIW, the SAI pump should only come on for a short time while the engine is warming up, but since my A6 doesn't have SAI anymore I can't say for absolute certainty.

Yes I am certain, a vacuum leak would be more of a hissing noise, this sounds almost like something is either slipping or lightly rubbing, and it is difficult to determine if applying the breaks changes the pitch of the noise because the noise is noticeably louder when I take my foot of the gas, regardless if the breaks are applied or not.

[brokenwrench]

To me it sounds most likely to be the TC/trans. I've had similar problems on other cars when the TCC wouldn't lock up/engage properly.

But just for the sake of ruling everything out, have you checked for metal shavings or un-natural deposits in your power steering system? The rack is directly under the firewall, and it's possible (however unlikely) that any noise in the system could be carried to the steering gear where you can hear it. Just a thought.


Cheers!

[A6xEnthusiast]

To me it sounds most likely to be the TC/trans. I've had similar problems on other cars when the TCC wouldn't lock up/engage properly.

But just for the sake of ruling everything out, have you checked for metal shavings or un-natural deposits in your power steering system? The rack is directly under the firewall, and it's possible (however unlikely) that any noise in the system could be carried to the steering gear where you can hear it. Just a thought.


Cheers!

Yeah I honestly was thinking the same thing. I listened to it again yesterday morning at idle and driving around and it sounded like something slipping/not engaging properly. Definitely most noticeable at first gear. I have not checked for metal shavings/un-natural deposits in my steering system but I can tomorrow and let you know what I find. Thank you for your response!