Oil pressure is low at tickover.....how low,,,, get then to check, probably using an external guage is best. Remember, hotter oil = thinner = less pressure at idle. So idle oil pressure will be higher for a cold atart.
So back to oil pump wear or blocked oil channel as mentioned... If the chain has stretched slightly, it will be longer, can they asses how much the chain tensioner moves to take up the slack?
It obviously moves enough when higher oil pressure forces it as revs rise, but is it making enough contact when idling, is there slack in the tensioner mechanism?
Could you just put a GTD badge on the tailgate and tell everyone, it's supposed to make that noise idling? (sorry!)