Noise and drivetrain play

Hello my 440 Belt makes a soft clunk at low speeds when accellerating and decellerating. The the isolator in the rear drive hub has a little bit of play, the swing arm has a very small amount of side to side play, I think belt is under the proper tesion. When I took the cover off of the drive pulley I put the bike in gear and there was quite a bit of free play there when I rocked the rear wheel. Primary chain and gear s are new. Is this noise normal or sign of too much play in the drivetrain? Bike has 13K on it.

I am going to remove the isolator form a parts bike later today and see if that helps. Any other suggestions?

I used to own the '81 edition of your 440, back when they were still 'newer' (mid eighties). I seem to recall there being some play in there, even then; I can only guess that as those rubber bits have aged and hardened some, the play would have increased... How much is too much? I dunno; I'd consult the manual to compare it to spec., or replace the rubber bits.