My ST-12 head is about 4 years old and has about 400 hours on it. The bearings have always been audible at low rpm but quiet running.
I just moved my Genset into its new home, a small wooden shed. Where restarting it, I was greeted with a horrible racket.
The noise screamed like a 30 HP blower in a factory, made my ears ring and drowns out the noise of the Lister.
When hand turning the genny, the noise was obviously coming from the bearing under the drive pulley.
So out it came, apart for the first time and voila...stuck big time. No way this thing would be an easy job.
It took all day making parts to form a bearing puller to get the brush side bearing out. Finally it gave up and popped out. The local bearing shop had nice German sealed bearings in stock for $45/each. It went together quickly and now runs quiet as a mouse. Feels like spinning a hot ear of corn on a stick of butter.
An interesting note is the bearing at the brush end, the smaller of the two, run warm, about 130 deg F while the drive bearing runs at ambient temp.
Suppose thats just the way it gonna be.