I will also jump in with the exhaust elbow
being blocked or at least restricted.
I have had this problem before and as a TEMPORARY fix, with the engine running at about half speed in neutral, tapped on the elbow
with a small ball pean hammer starting nearest the exhaust exit and working back toward the engine. This helped for a while by knocking loose a lot of the crap/rust/carbon. By starting downstream and working up most of what came loose was washed out the exhaust port.
When I got around to fixing it properly( you did see TEMPORARY?) I had to flush out my muffler
as a lot of the rust and crap was heavy enough to stay in the bottom of the muffler
Now, of course, y'all have made me paranoid that my water
pump impellor needs to be changed just as we are going on our vacation
. I may have to work on it before we go. Just to make ME feel better............m