I've actually solved this but thought I'd pass the fix on.

The dryer was working perfectly then my wife noticed a bump - bump - bump noise during a drying cycle. All the clothes were removed and it still made the noise when empty.
After isolating it from the mains, I removed the back cover to find that it only covered the heating elements and a radial fan blade.

Removing the top cover I was able to check the drum belt for wear and the drum by spinning it by hand. Everything fine.
I reckoned I'd have to check bearings but access to the motor, etc, looks like it can only be done from the rear. No removable side panels.

Note: When on the cooling phase, the noise goes away! Hmmm...

The cause turned out to be the rear radial fan blade assembly behind the rear cover, which was loose on the end of the motor shaft; there was noticeable play when you turned the blade while holding the drum stationary. There was a smear of grease on the shaft end which probably firmed up the fan blade when in the cooling cycle but allowed it to bump back and forth when the dryer had heated up. Tightening the bolt holding the fan blade to the motor shaft cured the problem!

Note that switching the dryer off before the cooling cycle has run can cause the heater thermostat to pop in which case it'll need manually reset (orange/red rubber pad in the middle of the rear case.) The cooling cycle is for cooling the machine, not the clothes.

Soadyheid, February 2016