Hey, I have a 3rd gen 4runner that's been making a vacuum sound for the past 2 years or so now.

Here's what happens (only when heat is on, I think the a/c too but I can't remember):

When I'm sitting idle I'll first hear a click sound coming from I think under the glove box. Sounds like a relay switch. Then the idle will go up about 100 rpms and there will be a vacuum sound coming from the same spot as the click sound. Then about 5-10 seconds later it'll click again and the vacuum will stop and the idle will go back down? This happens at almost every stop light. Anyone know what this is? Thanks for any help!

 

That is perfectly normal. Its your AC pump turning on and off. The newer systems are designed to dry the air when the heat is turned on, so your windows defrost.

 

Ok thanks a lot Cougar, glad to hear it's normal! I must not have noticed the first few years I had it.

 

If you watch your AC button, the light will click on too, indicating that the pump is on.