Intermittent strange sound on startup, ideas?

I think this has been going on since I bought my truck, so it must not be major. Anyway, sometimes when I start my truck, it'll take off and rev then start dropping rpms and I'll hear a "whonnk" from from the engine. It almost sounds like gears slipping or something. I don't remember it this winter at all, and I don't remember it ever happening in the mornings. It seems to be happening when my truck has been in the parking lot all day and is hot. I've been experimenting with it some lately and when I turn the A/C off then start the truck, it has not done it yet. But, if I turn the A/C on after starting and letting the rpms drop, sometimes I do hear it, but not as loud or long.

Any ideas? Belt slipping under the sudden load from the A/C maybe?

Great forum by the way. I've already changed fluids on my rear diff, transfer case, and engine oil; changed my sparkplugs and I have parts for the rear diff breather mod and oil for the front diff! All since starting reading here a couple weeks ago.

 

Its the fan clutch, and its normal. Makes that sound until it gets up to speed, changing the load on the engine (ac/no ac) will effect the sound somewhat. Should only happen when the engine first starts after its been sitting a while.

 

The fan clutch would do it in any season. When the silicone inside the clutch is cold(truck's temp gage is cold), the clutch is engauged. It stays engauged for a short time after startup, then freewheels until it reaches the engagement temp.
Until we get a better description of the noise and approximate location, all we can do is guess.

 

The noise is most likely coming from the clutch on the ac compressor. I've got a 2011 DC 4x4, and it's done it occasionally almost from day one. How I figured out what it was, I raised the hood and then started the engine and ran back around to where I could "eyeball" the ac compressor--and sure enough, when the compressor kicked in, it made the sound.

If it's any consolation, I've got a little over 3500 miles on my truck now and it definitely seems to do it a lot less than it did at first. Maybe the clutch has a break-in period or something.

Hope this helps.

 

Sorry, I just realized that the OP's truck is an '08. Maybe there's not a break in period for the ac compressor after all.

 

AC clutch makes sense, I didn't know there was an AC clutch.