Anyone have or can diagnose this problem?

When my truck is idling warm, every 20-30 seconds there will be this dull "thunk" that comes from somewhere on the truck. You can hear it and if you are totally still, feel it. It sounds like someone lightly punched the side of your truck through a pillow. I've gotten out before and crawled under it and heard it, but I have NO clue where it's coming from. It's loud enough you can hear it outside and inside of the truck, but it's not THAT loud. It almost sounds like something switching over, like for say the transfer case from 2wd to 4 but I never push anything! Anyone?

I'd bet thet is the AC compressor cycling. The compressor will run when you have selected the defrost setting.

This happens with the defroster/fan etc.. all off. Nothing is running but the truck itself.

i have that same noise, don't know what the hell it is

get out and watch the compressor even when off I think the cycle, may be wrong though

I think my truck does the same thing. On most newer cars the A/C compressor cycles periodically to keep everything lubricated internally.

Yes the AC compressor will cycle on/off even without the AC turned on. I believe it does this as a sort of de-humidifier to reduce cab humidity and as so fogging of the windshield.

you described it as a thunk. I would take that description and suspect that ypou mayn be hearing the fuel tank expanding and the side of it thunking in or out . Perhaps pressure from recycling fuel build and then is release on a cycle? If you had said a tick or somthing then i would suspect somthing turning on or off but a thunk is something moving.

My compressor also thunks, but I have never really checked to see if it does that when the ac is off.

I've experienced something similar... exactly the way you've described... not only in my truck but in my car... I was told this is normal... go figure.