'97 200EFI Wierd Sound

[zookimota]

I've got a '97 200EFI on my Nitro Bassboat and it makes the weirdest sound. Let's say running about midrange 3k to 3.5k rpm just cruising, the engine changes pitch constantly from an "oohhh" sound to an "aaaaaa" then back again. No change in rpm either way. The "aaaaa" sound seems pretty loud setting in the boat. A little change of trim will make it go away but when the boat settles to it's new attitude, it returns. Going over another boats wake gets a pitch change over each little wave. Sounds like a plastic rattle almost. Could the can be loose in the mid? It's been this way for quite a while, in fact was doing it when I got it over a year ago and hasn't gotten any worse or any better. Engine runs up beautiful at top and idle, just has this weird changing sound about mid range.
Any ideas?
I originally thought it was the jack plate, but removed it this year and noise is still there. I may have lost a couple of mph on top end without the jacker, but the hole shot is a lot better without having to mess with the engine height all the time.

[M. HARDEE]

Mercs have a normal harmonic HUM at those rpms or ya got ya motown cd player hook up to the tac . sound tape would be helpfull :D:D:D:) just my opinion

[jody504]

I had one the other day that had a funny sound in the mid the tuner was just about to fall out the bolts were almost out the plate was were out where the tuner goes it been rattling so long the tuner was in two parts.

[chad202]

That's the range the poppet valve open/closes. When it does, it goes from dumping water down the tuner to out the back.. Thats what you're earing as it will change the pitch.