weird noise on start up

i've noticed this for a while now. when i start my car up, pretty soon after the engine fires up i hear a loud thump or pop from the engine bay, kinda sounds like something is hitting the bottom side of the hood. i don't think anything is though because i don't see a mark. it only happens one time and after that i won't hear it again. it only usually happens after the car has sat for a while.

one bump to the top to see if anyone knows what it might be.

will check on the defrost, but i'm pretty sure i don't have it on.

all the hoses and connections are fine, the car drives great. no problems at idle and no misfires.

this only happens after the car has sat for a little bit. for instance, last night was the first time i had driven it in a couple days. soon as i started it up, i heard the thump. went to dinner, after dinner when i fired it up again, no thump.

car doesn't bog or act funny. beyond the initial thump, there are no other noises that i can hear that seem odd.