Mystery 93 Camry Starter Problem

Replaced my 93 Camry LE (100,000 miles) starter motor last year. A week ago the starter wouldn't turn. No loud clicking of the solenoid, or grinding of gear teeth, just a dull, almost unhead click each time I turned the key. Battery and alternator checked out okay, so my mechanic replaced the starter free of charge. Days later it happened again with exactly the same symptoms. My mechanic feeling he'd gotten a bum rebuilt starter, replaced it again, no charge. It has happened again today. A pal says it sounds like what happened to his 88 Honda and he believes it was the starter relay or switch that needed replacing. My mechanic is a great guy and I can't call him again without offering some help. I'd appreciate any advice or insight, failing an answer, even another online group I can pose this problem to. Thanks.