Might be your Secondary Air Injection pump. Mine sounds like a vacuum cleaner when it fires up.
I have an 05 GLI and have had this humming noise come on right after startup. I had my engine replaced a year ago, with a used one, and it was done by a small mom and pop shop. Since then I've had the humming noise come on right after startup that lasts a good 30-40 seconds and sounds like some kind of electronic motor under the dash area. It will stay on for that duration of time even while driving down the road. Then it shuts off. When stopped at a stoplight it will randomly come on after I release the brake pedal and continue through the intersection.
Does not affect a thing performance wise but I'm wanting to know what the hell this noise is. It is kind of annoying. I had noticed that it was worse last winter when it was cold out and the humming would last a couple of minutes.
Anyone have an idea on what it might be?
It really seems to have to do with the break pedal and when I push and let up on it it will make the noise. Sooo, to be more specific it does it in a couple of cases: 1. when first started up 2. when break pedal is engaged and released.
I think it's the SAI. Put your hand on it while the noise is buzzing. It's really hard to hear it with the engine literally right upon your other ear.
If not, then your ABS pump may be coming on because your wheel sensors are acting up. When you first start the car and while the car is still stopped in the yard, push your brakes down as hard as you can (as though you're performing an emergency stop maneuver.) See if your brake pedal vibrates. If this is the case then your ECU thinks you're moving and are trying to stop.
Read up on how ABS works. I read an article about it on www.howstuffworks.com about six years ago. After you do you'll see how this would kinda make sense.
I'm just throwing an idea. I may be wrong, but it sounds good. Try it. Do It
Good input. Actually, my wheel sensor has been on the fritz. The cluster has had issues with idiot lights coming up and I had a mechanic look it over a few months ago. He ran a test and claimed that a wheel sensor is not working according to the code. I didn't have him replace it at the time because he was working on something else on the car that's more of a priority.
My ABS and ASR lights are lit up and because of the sensor not working correctly from what he had told me. What's weird is that I had the sensor replaced almost two years ago. Would the SAI come with the used engine that replaced my blown one? At any rate, I'll check again but I think you may be onto something regarding ABS system being all messed up.