I'm not quite sure where to look for this problem, I tried search but couldn't come up with anything, and I wasn't really sure what to look for. So I'm sorry if I'm missing the obvious.

The problem just started, I left the garage last night, truck was fine. Visited a friend, left the truck in the rain. On my way home I noticed the problem.

So, the problem goes as follows: The engine is fine for five seconds, then I hear a something that sounds like something between bubble gum popping and a leaky faucet. Half a second after the pop, the engine rev increases slightly for about a second, then the sound occurs again and half a second later the rev drops back down. Exactly five seconds later the noise occurs and the cycle repeats.

It's the oddest thing, and quite unnerving when I'm trying to park and the truck suddenly picks up speed. It's not enough to be dangerous, nothing more than a low-speed collision, which is what bumpers are for, but it makes parallel parking difficult, and I don't like the feeling of losing control.

So, any ideas?

This is out of left field, but could it be your A/C compressor coming on and off?

I don't think so. I mean, I guess it's possible that the compressor is turning on every five seconds for exactly two seconds, but since the A/C is off, it'd be one heck of short circuit.

Yea, I forgot to look at where you were from, I am sure the A/C is not getting much use this time of the year.

The compressor still kicks on and off when its in defrost mode. Even if you have it set to heat. That way it doesn't sit all winter not being used. With that out of the way, possibly IAC? Try Cleaning it, Search for IAC or Idle Air Controler.

Is this really something that could be kicking on every five seconds like clockwork? Not just that, but I thought the compressor made the revs drop slightly. I think I would have noticed if the truck was constantly lurching at stop lights all summer long.

well in the summer ur compressor will stay on longer but if the car is defrosting it will click on and off to take the watter out of the air so the windows wont fog up so today when u drive dont put the vents on at all leave everything off see if it stil happens

sry i cant be anymore help..i may be wrong about the taking the water out of the air thing but tahts what house acs do

So look at this, after 10 years of driving around in this thing, this it the first time I've noticed it. Turns out, yes indeed the compressor is just kicking on and off. D'oh, I'm an idiot.

Thats weird thats its kicking in and out every 5 seconds though. That seems too much. Mine normally kicks does it for a couple mins for it's cycle. Just be glad its not like my dad's work truck(dodge). When the comprossor kicks in it makes a loud bang and practically stalls the truck, and then bangs out.