Hi guys,
My Car: 1999 A6 (1.8T) with 260,000 km (manual transmission)
At the neutral gear position, I get the following sound whenever I rev the engine (I don't hear it at idle). The sound is not very loud since you can' hear it when you are inside the car - I am praying to god that it's not a rod knock due to a spun bearing.
When I first heard the noise the car would shake and shudder, but once I got on the highway the shakes disappeared and it ran fine. I don't get the shakes anymore at low speeds as well just that annoying knocking sound. I recently topped up gasoline at a new pump, could bad fuel cause this?
Does it sound like it's coming from the bottom end of the engine? How is your oil pressure?
https://www.youtube.com/watch?v=wmtBqNnnvrs
No trip in low oil pressure (I don't have an oil pressure gauge) and yes the sound does appear to be coming from the bottom. However, it does not sound as loud as what's shown in the video you posted.
There's not much else that would be making that kind of noise coming from the bottom of the engine, but noises do telegraph all over the place and it can be hard to tell their true source. If you're lucky it might be an accessory drive making the racket. However, you might want to check oil pressure by hooking up a mechanical gauge.
Good news,...the sound went away. My mechanic says that it could have been poor-quality gasoline (which has been a widespread issue in my country). Do you think I should change the fuel filter?
I'm not sure how bad gas would cause that kind of noise, it doesn't really sound like pre-ignition due to low octane. I'm not familiar with that engine though, hopefully that sound will stay away! If contaminated gas is suspected changing the fuel filter is probably a good idea.
Just did an oil change. Thank god no bearing material. *Photo attached after passing oil through a strainer*
The dealership says it could be the flex plate. However, they can only confirm if they pull the transmission out.
A loose or cracked flex plate can certainly make a racket. (The engine would have cratered by now if it was a rod knock, you first brought this up over 6 months ago.) Isn't there an inspection cover they can use to check the flex plate bolts? I'm not familiar with that model but there should be one. The flex plate is normally unbolted to remove the transmission. That's done by accessing the bolts after taking that cover off, rotating the engine to get at each bolt in turn. (Unless you remove the trans while leaving the torque converter hanging on the engine.)
This video is of a completely different vehicle (an old Jeep) but the idea is the same:
https://www.youtube.com/watch?v=5Qq8r9qNfJk
Update
Hi guys,
the problem has returned, but now based on the following details:
1. Engine needs to be warmed up, you will not hear the sound on a cold engine (after about 6-8 mins of city driving).
2. Only at high revs.
3. No loss in power, but the sound frequency and intensity increase with higher revs.
4. Present in neutral
5. Engine oil + filter was changed and no metal or bearing material was present in the removed oil.
6. Sound is more of a scraping sound, that feels irregular in frequency.
The dealership says it could be the flex plate. However, they can only confirm if they pull the transmission out.