I had a clog in my cooling loop. I didn't know it was there, my dealer found it and took care of it. Last fall I was running the boat when there were a lot of leaves in the water, I must have pick up a bunch through the in take.
I didn't notice any difference in the Exhaust note, but I have a Fresh Air Exhaust system, I don't hear any exhaust any way.
Your impeller could be bad. If you just bought a boat and don't know when the last time the impeller was changed you should look in to replacing it. I put on about 200 hours a year so I replace mine every year.