05 M620 grinds under engine braking

[Streeticus]

Under engine braking at any speed in any gear it makes a grinding
noise. It's not very loud and other than the noise i haven't noticed
anything else as far as the way the engine feels. I, nor anyone else I
know can figure out what's causing it/how serious it is.

I sent an email to the guy who does my service, but I figured that I'd ask here too.

It has 10400miles on it, had it's last service done at 8400.

[DarkStaR]

Could it be the chain?

How is the chain tension/alignment?

[Streeticus]

The chain is tensioned Right, but slightly rusty and due for replacement within the next week or so. If the chain could be causing it, that's probably what it is. This just gives me an excuse to go down to a 14T in the front

[Latinbalar]

could be the slipper? From another post here there are instructions on high to tighten it.