curious about noise A/C related

[pamoffa]

My noise only happens once a day it is when I turn on the AC there is a loud thump I can feel in the floor under my feet, I can turn the ac off and on all day and it wont happen again, but the next day first start up and use of the ac it happens, my question is do these trucks have a loud flapper door that is known to snap closed that could be loud enough to do this? I really dont think it is compressor related and it is a very reliable noise.