Metal "clanking" noise in Yamaha sterndrive exhaust

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ryno1234

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I have a Yamaha sterndrive (a relic made for just a few years in the late 80's early 90's) and I've managed to keep it running for the past 20+ years quite well.

All the time I've owned it, there has been a randomly timed "clanking" noise that I always assumed was the gimbal bearing. Happens roughly every ~1-3 seconds sometimes more tightly grouped sometimes spaced out a bit. I wanted to replace it at first, but after time, it just continued to work without an issue. Pair that with the fact that parts are impossibly hard to find for this (and expensive), I had ignored the issue.

Fast forward to last week when I pulled the boat from storage for the season - I noticed that there is a very loud clanking / rough / rotational noise from the rear of the boat near the transom while its running (regardless of if its in forward / reverse). I figured again that it was the gimbal bearing that the problem was that it finally gave out.

I ordered a new gimbal bearing.

While my order was in transit, I removed the outdrive and started the engine without it on. I'm still hearing the original "clanking" noise I've heard since day-1 of owning the boat despite the gimbal bearing not even being engaged. Clearly this isn't the issue with the "clanking" noise. That said, I'm not hearing the newer "rough" / rotational noise I was hearing previously. The clanking noise can be heard through the exhaust bellows. This completely baffles me.

What could these noises be? I assume the rough / rotational noise is the gimbal bearing, but the clanking noise that I've heard all along is a mystery to me. I've rebuilt the engine back in 2001 and really it doesn't have that many hours on it all in all. The only thing I can think the clanking noise is, is something within the exhaust manifolds since this type of noise wouldn't be something that would come from within the engine itself.

Any insight?
 

Bondo

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The clanking noise can be heard through the exhaust bellows. This completely baffles me.

Ayuh,...... That's the remnants of an exhaust shutter,.....
 
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