Got a new noise on shut down

So I was at Chris's in Suffolk, VA getting a KDMax tune and on shut down the was a "POP" when he shut the motor off. I'd never heard it because I was always in the cab and my hearing sux. Chris said he never heard it before on a Taco. I finally enlisted my son to help and we narrowed it down to Secondary Air Pump in the passenger front corner of the engine bay.

The noise sounds like if you pinch a partially inflated balloon, pull it back and let it go. Best analogy I can come up with.

I installed an LCE Long tube header about 3 mos ago which works great but I don't know if the noise was there before then or not. Again from the cab you (I) can only hear it if I am paying attention to it.

I have no MIL (CEL) Lights.
The truck runs well except for an intermittent dip in idle speed at stop lights which I "think" is the AC compressor kicking on and off.
With 75% highway driving I get ~20 mpg or 380-400 miles per tank.

I've looked at all the threads here that I can find and am not seeing anything similar.

Is this a symptom of imminent doom?
Common, don't worry about it?

Dunno squat about air pumps, have a 99 and a 77 this is my first vehicle manufactured in this century and I'm learning as fast as I can.

Appreciate thought and ideas.

Cheers,

Doc

 

The third gen has an airbox "pop" you can hear only from the outside that happens intermittently on idle with the AC on or when you turn the steering wheel. It'll sound most notable on the passenger side (airbox, obviously). You could try turning on the truck and having someone turn the wheel lightly to the right and left over and over, you'll hear a "pop" every time the wheel turns. I'd check to see if that's what you're hearing.

If so, completely normal.

 

I know when I turn off my truck, there is a buzz sound coming from somewhere that can only be heard if I have the windows down or door open.

 

That might be the evap/emissions system purging the fuel lines. Heard it the first time while sleeping under my truck in the cold. Scared the hell out of me, but it's normal

 

Sleeping under your truck?

 

It was a cold night and I wasn't prepared, nice and warm under the truck though

 

Thanks for all the ideas guys. No joy on any of them. It still happens after you turn off the key and the engine winds down then "Pop". About 90% sure its the secondary air pump. Anyone know a way to test it?

Cheers,

Doc

 

not a bad idea. sometimes we get out to the woods late and can't find a proper place to setup camp if the weather gets bad. might have to try that with a tarp over the truck and parked over a small depression to increase headroom.