If it is an impeller problem, I'd suggest running it off a bucket fed by a hose rather than just stuffing the hose in the raw water line. That will let you see first hand if your impeller is drawing water efficiently.
To run your boat on a bucket, get a 5 gallon bucket and set it on the port side of the motorbox. Remove the raw water hose from the thru hull fitting (aft of the engine) and stick it in the bucket. Fill the bucket about 3/4 full and crank the engine. If you can't get your rw hose off the thru hull fitting, you can get some flexible pvc hose (1.25" ID) and attach one end to the hull side of the transmission cooler (lower port side of the engine along rw hose---little black cylinder with tapered ends) and drop the other end of the hose in the bucket. I had a pretty stubborn thru hull fitting, so the latter method was the one I always used. Make sure you put a hose clamp on that pvc hose so it doesn't leak.
I have no idea what the sucking noise is.
Previous: 1993 Prostar 205
Red 1998 Ski Nautique, PCM GT40, 310 hp, , Acme 4 blade, Perfect Pass SG/Zbox.
Be kind. Have fun.