I own a 01 L with 12,000 miles on it. When I am on the highway and turn the AC on I hear a loud thud, which sounds like something expanding under the hood. Any idea what this could be and why it makes the noise?

 

If you think the noise is inside the cabin, e-mail me at wydopnthrtl@yahoo.com (experienced HVAC engineer)

WOT

 

i would guess either you are hearing the compressor engage or you are hearing the vent doors behind the dashboard change positions.

 

Yeah, I thought that also, but it makes a sound like when you hit a small varmint and it bounces underneath. Make sense?

 

your first post says it does it when you are on the highway, is that the only time it does the noise? it wont make the noise if you are stopped, then turn on the AC? possibly driving along at freeway speeds and turning on the AC gives the compressor more of a shock and it makes the engaging noise louder than if stopped?? just a suggestion Mike

 

I think I know the sound, and I think SLP 97 Camaro guy is right, I ocassionally hear the vacumn from the engine causing the vents behind the dashboard to open or close I guess, depending what you have it set on, my guess is that with you running at highway speed the vacumn used to open the vent is much greater allowing the vent to open harder/faster causing the thud you hear. Just my .02

 

Your fan isn't on "high" when you first put the air on, is it? That will cause the noise.Before shutting my air off, I always bring the fan speed down to one.Its prevents vent problems later on.

 

Just don't turn the a/c off till winter