2.8 30v, can't hear it when the engine is idling, but when I am driving i hear a semi high pitched noise, more prominent at lower rpms while on the throttle, but I can't hear it on free rev with clutch in or in neutral. I can hear it more if I'm going over a bridge or driving on a road with a curb to my right, but it's way more high pitched. I thought it was the clutch at first, but i'm really not sure. I also noticed that the rpms seem to fumble a bit at idle, causing the car to shake (not bad) but I'm not sure if it's related. How can I better diagnose, anyone have an idea what it could be?

 

anyone?

 

Best advice and cheapest is to go to autozone or car parts store you prefer and get 4 ft of vacuum line (89 cents a foot) and do all the vacuum lines (I personally do not like the approach of buying new parts and guessing but the vacuum lines usually need replacing anyways). A "hiss" is usually a vacuum line and will definitely cause your idle stumble. Its a starting point at least for now. Also something to keep in mind, the idle issue and noise may not be together, you may have a dirty pad/bad wheel bearing causing the noise only while driving and the idle issues is not connected. I know my a4 did the same thing and I found it was the RIGHT rear pads. Good luck keep us posted

 

It's strange what noises resonate off highway dividers and in your case curbs. I was thinking wheel bearing too, but that's usually a groan and not high pitch. But you should try and see if it lessens or stops if you are going down a straight road and turn the wheel slightly to the left or right. That's an indication of a wheel bearing going bad because you are changing the load contact point slightly. Other than that, I don't know what to say.

 

Thanks for the suggestions. I think it may have something to do with the clutch or transmission. When I push in the clutch and put it in neutral I don't really hear it. What would that indicate?

 

I think what I'd check next, since it doesn't make this noise with the clutch pedal depressed (since that disengages load applied to the transmission in all gears, you are coasting) would be if you hear this noise in that same low rpm range in all gears or one gear, foot off the clutch of course. If you can tell changes in the high pitch noise note the circumstances and see if somebody here that works on transmissions can help out, or call up a good shop in your area for help in diagnosis. In my limited experience, in a 4 sp trans, in 4th the power flow is straight through the trans from input shaft to output shaft and very limited side load is present inside the transmission. So those other bearings don't get any pressure applied to them. But in 1-3rd the trans is running a ratio other than 1 to 1, meaning for one turn of the input shaft you get one turn of the drive line (for a rear wheel drive car in this example) so a side load is introduced in the guts of the trans for gears 1-3rd and if a bearing is going bad then you hear a groaning or some other noise in those 1-3rd gears. If you heard that noise in 4th it would most likely be the input or output shaft bearing. My knowledge of transmissions isn't great so check this info I'm giving, but it should give you an idea of what to listen for when driving your car, and why you are listening for noises in different gears, RPM's, and throttle input to change that noise might play into helping you diagnose this problem.

Otherwise I'm with Dozonti, and check into vacuum leaks. Those can cause hissing also, but will change with throttle input, low rpms and wide open throttle make for almost zero vacuum in most of the intake system, so you shouldn't hear much of a hiss, but 3000 rpm and on decel in gear will generate 18 or more inches of vacuum and should make a vacuum leak hiss audible. Sometimes in a J boot that goes from (which doesn't get manifold vacuum so would probably only hiss on higher air flow velocity situations since it is BEFORE the throttle body), the MAF meter to the throttle body, if the engine moves around from shifting gears or abrupt accel or decel and that noise changes, that tells you the split only opens intermittently when that boot moves to open the split in the boot.