I have a random problem that seems to be getting worse. Starting a few thousand miles ago every few miles on the highway I would hear what sounds like a skipping, or a hissing sound. It seems to be coming from underneath but who knows sometimes it sounds like it's coming from the passenger area too. It is a sound that is hard to describe. A very short hiss maybe for a second and then no noise. A few miles later I hear it again.

Now the problem is occuring at lower speeds - not on the highway but just anywhere.

One mechanic I use said it sounds like it could be a valve just blowing off. There is nothing obviously loose or damaged on the vehicle. Tires are fine, shocks are not leaking, no leaks anywhere. My truck has about 88,000 miles. Anyone else have a problem like this or any ideas?

The only thing that rings a bell to me is when the A/C turns on. For a quick second or two, like you described, it will hiss under the passenger dashboard and then go away. Try putting your head down there while the runna is parked and turn the A/C on and listen. If thats not the case then I have no idea.

if it is related to the AC, is it something that would start to occur say 7-8 years into the 4runner's life? Because it never used to do it. Could it be an ac part wearing out? It's really hard to describe when I say a skipping or hissing sound i wish i could play an audio tape of the sound for everyone to hear it. One thing about the sound is that it occurs at random intervals, but they seem to be getting more frequent.