You have not indicated manufacturer or model most likely blade hub attachment loose would be my guess !

Hey Tom , on the same page bud . How are you was not sure if you were still on the forum !

I get this when the grass is too high and I plow through it anyway. Happened for years on my old Toro 20007 with no adverse mechanical effects. May have worn out my belt more quickly but that's a simple replacement.

Your correct Woody , the death rattle can ocurr when you exceed the cutting rate or rate of cut of the mower . Tall or wet grass can easily contribute to this situation !

Can you confirm the noise is coming from the motor and not from underneath?

If motor, I would bet the noise is from the moving easy start parts inside the engine. They move to open a valve earlier(or whenever) to make it easier to pull start. When the motor reaches a certain speed and up, these pieces swing out so only the normal camshaft opens the valves normally.

The motor slowing down to near borderline speed between enough for them to swing out and back in thus rattling.

Up up how's it going ? Not sure of the easy start components as a contributing cause . However often in these circumstances the governor cannot overcome the load applied to the engine and addition fuel cannot be burnt sufficiently to overcome this load . Therefore you notice a form of preignition within the engine where unburnt fuel is being burnt out side the combustion chamber timing sequence. This can cause severe engine damage and should be avoided ! 🍻

ssewalk, from the few times I've opened up my motors, I went looking for any loose part while it was open and the only "loose" part is the easy start's moving parts. All of the stuff I have used that have easy start made the same noise when i hit really tall grass or pile of leaves, etc. None of them ever blew up reinforcing my guess.

BTW: is it me or has this forum pretty much died??? Nobody posts anything new here. I just look once in a while...

Not saying some form of easy start mechanism could not contribute to the op,s noise complaint , just that I have never encountered one . As to the forum , not near as many experienced wrench heads as we previously had a few yrs back unfortunately . Anyhow good to see you are still kicking !

Need know your machine model.