Tumble Dryer Alarm Clock

It looks like my tumble dryer has an alarm built in to it that wakes me up about once a year. Last year it was a clacking clanking scraping sound (see here).

This year it's a thump thump-thump sound. (Sort of D in Morse code). OK, it's 1:30am so I don't get up, but try to sleep through it. After a couple of hours the dryer finishes its program and I get some sleep. Five o'clock comes round and I get up and investigate.

I've forgotten how to get the top off properly (just undo the side screws and the top comes off to the front. Don't undo the rear screws like I did. Again). To get the sides off I need to take the front panel off and to do that I need to take the bottom vent off. I forgot that it slides on these clips:

I finally have the top and two sides off and can now look for the D-thump. Turning the drum manually I can just feel a bump now and again. The rear bearings look fine, so I have a look at the rollers. They are fine too. Then I notice the drum has some black marks where something has stuck to the track the rollers run in.

I find that there's a big blob and several smaller blobs, which probably accounts for the D pattern of thumping. I scrape the blobs off with a wooden stick and re-assemble the dryer.

A quick test and the D-thump is gone. I have no idea what the black blobs were. At first I thought it might be material from the rollers, but they look fine, with no obvious missing material. Maybe something fell onto a roller and ended up smeared around the drum?  I have no idea, but the dryer seems well again, which is good.