High pitched straining noise? help?
 

I'm fairly new to the Audi scene, I have a 1998 manual 2.8L Quattro, 124,xxx miles. When I turn the car on after it's been sitting for awhile (few hours) there is a high pitched straining noise. It sounds very similar to an electric motor on its last legs? I don't really know how else to describe it, also it seems to be coming from the drivers side under the hood. I haven't been able to locate it yet, becasue it only lasts for about 1 - 2 minutes max. Also once it stops if I turn the car off then back on, its no longer an issue. It only comes back after the car has been off for a few hours. There have been no performance issues related to the noise(yet), it just sounds very sketchy, like somethings going to break soon. Any ideas? It started about 2 months back. Thanks

Sounds like it could be two things. My first intuition tells me its just the secondary air pump clicking on. That pump is designed to run for the first couple minutes you start the car when the engine is 'cold' (specifically, I believe this is something along the lines of the coolant temp being below some specific temperature) and the function is purely for emissions, it pumps a little extra air into the exhaust mani to make the post-cat O2 sensors think the car is running rich so that the warm up stage doesn't spew as much unburned fuel while things are getting up to normal operating temp (at least that's what I've gathered the function to be, if anyone else can correct me, I'd like to know).

My second intuition however, is that it could be something to do with the power steering, since you said it was the driver's side and the SAP is on the passenger side directly below the air box. The power steering pump doesn't typically make any noise just by starting the engine. If you are low on PS fluid, it will whine when you turn the wheels, but it should be relatively inaudible at idle and no motion of the steering wheel.

That said, I would put more money on it being the SAP pump and maybe just sounds like its on the driver's side. My recommendation is to let it sit long enough that you know for sure it will do it, pop the hood and start it and listen closely to the passenger side of the car, right on the front corner to see if it sounds more like its coming from the SAP, and if it doesn't sound very strong, move to the other corner on the driver's side and see if that is the side it's on, that's where the PS pump is (IIRC)

Good luck and welcome to the forums :)

Thanks for the quick reply, I should have added this, but the power steering was making some noise during turns when I first bought it, but I topped it off with fluid and that fixed it. Also, the noise is more towards the back of the motor rather than the front, so I don't think its the power steering. If it is the secondary air pump, would anyone have a guess at cost of replacement? I'm taking it in to a friend who is a mechanic this Friday so hopefully he will be able to figure it out. Also I have been meaning to pop the hood and take a closer look, but good 'ol Wisconsin winter just started up and don't really feel like freezing my butt off ;)

I hear ya about the cold, it's been a pretty consistent -10c or so around here lately. I've had a taillight out for almost a week now and have had no motivation to stand out in the cold even for that 5 or so minutes it'd take to change it out haha

With regards to the power steering, please please tell me you didn't just go to autozone and get some power steering fluid... Our PS system runs on mineral based fluid, typical auto parts stores don't have anything that will be compatible with it and if its not mineral based, large problems result. This could well be your problem if you didn't get it topped off at a dealer or Audi certified mechanic or order the correct CHF-11 online.

If its closer to the back than the front, I wouldn't put too much money on it being the SAP in that case, the SAP sounds literally like a mini shop-vac running and is very easy to pick out as being on that front corner.

No I had an Audi certified mechanic do it when he was inspecting the car after I got it, so that's all good, but thanks for that tip becasue I wouldn't have really thought to check if it had to be special stuff or not.

That being said tho, it really does sound like a mini shop vac :(, but I could of sworn it wasn't in the front the last time I looked... I just went out to start it, but I drove it this morning, so it didn't make the noise... I will check again later and hopefully have a more accurate location.

Ok... so I don't know what I was thinking earlier, but the noise is on the passenger side, in the front. So with that new bit of info, any new ideas what it could be? I did take it to a mechanic but like I said earlier, once I drive it the noise will go away for a few hours. So the mechanic didn't find anything wrong with it...

Front passenger side sounds like the SAP is the more likely culprit. Is your check engine light on?

Nope, everything else is normal except for the noise.

Then there's nothing to worry about, the pump is functioning as it should and everything is running normally. The sound you are hearing is normal for the SAP, it is nothing shy of a typical vacuum pump so that's why it sounds like a shop vac. The reason you don't hear it very long is because it's designed to only click on during a "cold start" for emissions purposes, as I recall, it shuts off when the thermostat opens after the engine runs for a couple minutes, and it takes a few hours for the ECU to register the coolant temps as "cold" once again so that's why you don't hear it upon restart after its only sat for a couple hours.

As long as you don't throw a CEL with a code pertinent to the secondary air flow system, there's nothing to be concerned about. The sound may resemble that of a pump being on its last legs, and maybe it is, but what you'll end up doing is replacing the pump now cuz it's "on its way out" or you can ride out having to buy that $350.00 part as long as you can until the CEL pops on for incorrect secondary air flow and you are forced to get the new pump. One way or another, it'll be a new pump, but it won't hurt anything while it sounds like its on its last legs, and it won't do anything to the car other than throw a CEL when it does go, so why pay the money now to fix something you don't need to worry about for a while?

The SAI is actually meant to warm and provide air for the catalytic converter. The rear O2 sensors help determine whether the system is working correctly or not.