I had an odd noise on startup after sitting for a couple of days an odd "light thud", lasted maybe 10-20 seconds tops Stopped the engine and restarted and it was not there, started again this am no noise. So far this is a one time thing but I am a touch OCD

Checked the oil pressure good at 54 psi on start
I did notice what seemed to be some small bubbles on the dip stick, they are not there this am
No ticking of note when running
Running 10/30 synthetic with about 600-700 miles since the last change this is teh second oil since my head gaskets an it is due for a change to summer weight
Mobil M301 oil filter

Only thing of note is the day previous I drove about 6 hours in largely stop and go traffic, engine temps were fine 197 at the highest

Any suggestions ?

 

Exhaust leak? When I had my leaks they always presented as low range frequency like a thud. Easy to hook up up blower to the tailpipe and spray soapy water down the whole exhaust and look for bubbles.

 

@cvhyatt good thought, it was more like an exhaust pipe hitting the underside sort of thump. I wonder if my one cat coming a part.