A kitchen-mystery, solved...

...at my one back-alley cottage, in the (college) town that I now live in:

For a bit this summer when I'd open the door to my refrigerator, there'd be this loud THUNK sound that would like go through the wall standing right next to it, that it's tucked up against.

But, this would only happen sometimes.

Finally, I figured out what it was -- since the refrigerator door opened up to the right, right up against the wall and the window there, if I had the Venetian blind there pulled down so that the bar thing at the end of it was just even with the window sill, the outer corner of the refrigerator door as it opened would increasingly press it against the window sill until the plastic in it shifted and it popped up a bit, the resulting compression and decompression making a loud THUNK sound that reverberated through the entire wall.

So, I started making sure that whenever I pulled that Venetian blind thing down, I didn't leave the end of it at the same level as the bottom window-sill.

And, since then, it's never happened again.