Hey guys! I've done a lot of reading and can't seem to find any threads that describe the sound I'm getting. My truck is making a strange rumbling sound about 5 or 10 seconds after engine start. It lasts for about 2 to 4 seconds then goes away. I've never heard it while driving, only in Park after starting the engine. My truck is a 2001 1500, with a rebuilt engine. It made the sound before the rebuild. The transmission was rebuilt 20k miles ago. I'm pretty good at troubleshooting but this one has me stumped. It doesn't happen every time I start the truck, about 30% of engine starts it happens. It always catches me off guard. It makes it difficult to troubleshoot because I have to get lucky to replicate the sound. I think it's coming from under the hood. It's done it for a long time, it used to be subtle but last night after a warm engine start it was loud and longer than usual. It think it does it more on cold starts but occasionally on warm starts.

I've done a some looking around to see if anything is out of order. Everything seems fine. Fluid levels are good, all the engine components spin freely, I get good oil pressure. I'm pretty sure it's not the engine. My truck runs really good. I get about 17Hg of vacuum when idling. I've had this truck for 2 years now, the only 2 issues this truck has always had was the rumble and the air vents get weird under acceleration. When I accelerate (w/ medium throttle) and I have the air set to blow out of the dash vents it switches the air to blow out the defrost vents. When I back off the throttle the air vents switch back to dash. It's most likely not related but its an issue this truck has had as long as the rumbling sound.

Down by the passenger side headlight there are some sort of components that look like evap stuff. I hear a hissing coming from that area after I shut the truck off. Could that be related?

I'll do some more investigating and try to replicate this sound.

 

The vent issue is called "Wild Vents" and there is a sticky on it. Usually just changing the check valves fixes it but it didn't fix mine. Some have to install a vacuum pump.

The rumbling, could it be something vibrating

 

Wild Vents sounds appropriate. haha Thank you for the reply. I hate how my vacuum lines are set up. It's a mess of lines with a series of T ways and zipties. I wish it was setup to have 1 vacuum line from the intake to a vacuum bank or something like that, then everything else hooks up to that. I'll do some more research on that.

I managed to figure out rumbling has to do with the AC compressor kicking on after engine start. It doesn't happen when cycling on and off during normal operation, just on initial start. I see the AC lines jolt a little. I'll try to find some more time to figure out what is actually happening. Maybe employ a friend to start it for me while I look.