Mixing elbow is usually four bolts and a hose that is clamped on, sometimes it can be tough to get to, but level of difficulty wise if you can change a fan belt, you can R&R a mixing elbow, there is a gasket
on mine, I put a little grease on it and re-installed and had no leaks
. Unless the elbow is clogged, there isn't much pressure in an exhaust.
Growth on a prop can absolutely cause this too
How many hours on the engine? Usually but not always a "bad" injector causes a knock at idle.
Majority of the time it seems what you describe is most often a blockage in the exhaust.
Worrisome thing to me is how do you know when an elbow is going to fail? Some when they rot
away inside can let salt water
back in through the exhaust and cause engine damage, depending on design, not all designs apparently can fail that way.
If there is a mechanic available with experience with your engine, it may be worth buying
an hour or two of his time.
Filter is easy to check, remove it and if problem goes away, it was the filter, but I doubt it is the filter. Do you even have a filter, or do you have a screen
that catches big pieces?