Can you feel it on the floor board?

No- Can't feel it on the floor board.

Ok, totally stumped...

Since it starts at a predetermined time after startup, I'm betting it is temperature related! Like the heater core circulation etc. Or something silly like the Air temp door flapping back and forth etc..Something Crazy !

Dude chill out. it was just a joke...he wasn't being serious so no harm done.

Not trying to freak you out, but is it possible that its something to do with the flywheel/torque converter? I've read of quite a few people having some issue with them where something or another breaks and causes a knocking sound thats really hard to diagnose. Hopefully someone who knows the specifics of what i'm talking about can chime in.

Thats the only thing I can think of based off the symptoms.

This knock didn't appear all morning. However, it came back shortly after noon today. I hear the knock ONCE when I turn the ignition on (Not turning it completely over). It really sounds like something opening. Any Ideas? Thinking my next step should be a vacuum test no?

Start the car up. When it knocks open the hood. check the passenger side firewall. There is a vacume line distribution block up top by the ac stuff. It has a electrical connection too. Feel it i bet you can feel the thump as you hear it as well. It does it on my 99' nothing to fear. let us know.

You are a HUGE Help! That Solenoid was smashed down (Bent Bracket) and resting on a metal ac line. It does indeed thump/clunk. However, I do not notice it any more since I replaced the bracket and rubber grommet that holds it. Thank you. My check engine light did just come on after I unplugged the solenoid to move it. What shall I do? Is there a certain progression/key on/off sequence?

Thanks everyone

Matt

hold your trip/clear/odometer button down and turn on the key it will go through a check and should clear it ......its a mopar trick i learned......if not go to autozone or oriellys they can tell you what it really is

also the rpm change when the defrost is on is the a/c compressor cycling. It will do that even with the temp control all the way to warm. Something about removing moisture.

ProblemS fixed. Thanks for all your prompt help.

glad i could help.

The Air temp Door !! Flapping Open/closed. ??

It is right under the passenger side dash !

oops