Funny the bit about the acoustics. As part of my annual service I checked the belt and it was a bit "slack" I nipped it up with the usual caveats and a bit of twisting with my thumb. When I started the engine there was a small bit of "thrumming" for want of a better word. I ever so slightly eased the tension and it went away! I have noted in the past that if you overtighten them, they do "sing"The M80 looks very similar to my MD22, aka M50. Our boat and engine were launched in 2001 and the timing belt never changed until I've done it last winter. Unknown engine hours, but quite a few. Belt was in good condition, but like on skipper_stu's engine, the idler pulley (changed along with the belt and the tensioner pulley) was seeping grease. Wouldn't worry unduly, and you can inspect the pulley condition visually by only removing a few bits - should give you a hint of the engine hours too.
Doing it is a few hours of work. The Haynes manual has useful advice, as has this article, and probably skipper_stu
Since I couldn't lay my hands on a belt tension gauge in Portugal, I did some measuring, counting and a bit of math and acoustically checked/adjusted the timing belt tension with the help of a free guitar tuning app on my phone. This was fun and worked very well. Apparently that's how it's done in workshops these days.
