Has anyone replaced the rubbers in there harmonic balancer?

[Bucko]

I've been getting a slight engine knock recently (just started this year).  The knock is not there at idle or under power or when decelerating.  I hear it sometimes when compression braking and then give the engine just enough throttle to 'come off' compression braking but not enough that I'm applying power.

Anyone else experienced this and if so, did refurbishing the balancer resolve the problem?  Are any parts required other than the balancer rubbers?  I see some online repair posts referring to a 'bushing' but I can't find that on the parts fiche.  Thanks for any advice.