D85 Botvac is working fine, but now has a loud knocking/clunking sound when maneuvering. It is intermittent. I service...

You can buy parts from this website. (China) https://fqparts.com/ I got a new bottom cover and keyboard (after my kids had stepped on it). I looked for a long time before I found their business. Not even Ali express has this cover. The parts were spendy, but they negotiated for a deal and will ship out a package from the PRC in a couple of weeks. Payment is Via Paypal.

So the clunking turned out to be the brush motor becoming loose from the brush gearbox. I removed the gearbox from the D85 and put blue thread-locker on the motor mount screws. I also added a thin nylon washer between the main transmission gear and the gearbox. Everything is perfecto now.