My guess is that either something is rubbing or maybe the reed valves are leaking back a little. I think I hear what you mean by sounding like a horn. It’s kind of squeaky. Given the long history of twos being worn out and/or having bad bearings, I might go with that Idea, too. In the meantime, can you run it without the recoil housing? Maybe start it with a drill motor? That would eliminate that as a problem. While off, pull the flywheel and look for rub marks? Pull the lower unit, put it back in the tank and start briefly to eliminate the lower unit. (Sometimes sound from lower units telegraphs up like you wouldn’t believe.) Then make sure the reed valves are seating well…..maybe even do that first…

I think the squeaky horn sound is the reeds. I have one of these and it makes the same sound at low idle. Thats what I figured it was. Maybe I’m wrong, but if I am I’d like to know what is making that sound also.
-Ben

I’ll be interested to see if changing out the reed plate eliminates the sound or just changes it a little.
-Ben