Each year I service my bottom brackets a couple of times with a full cleaning and a lube with good grease. Lately I noticed a rhythmic clunk, clunk (sometimes sporadic) from a used bottom bracket I installed in a rebuild project. I opened it up and one single loose ball bearing was out of the cage. Ordered a new bottom bracket set and all is well. My 2009 Schwinn developed a slight clunk around the same time. Inspection revealed three escaped balls! That Schwinn probably has between 3,000 to 4,000 miles on those original bearings as I have averaged 10 miles daily for about the last 3 years. So another bottom bracket set install and what a difference. FYI I used a cheap WALD set from Amazon. I think stuff like that slips up gradual and you don't notice it. Just wanted to pass on my little victory. Part of it came out of pandemic cloistering and a desire to do something productive. Both bikes spin so freely they feel like new with my flip flops on the pedals.