Hey, thanks for the suggestion, though I’m hoping you’re wrong 😉 Haven’t had anode failures before as far as I know so I’m hoping that’s not the case here. (Major pain to remove it anyway due to tight installation.)
Anyway, went ahead and cleaned it out and replaced the elements. Lower one coated with the usual deposits but I’ve definitely seen worse. Only real point of interest was that the ones it came with are not the ones listed as correct parts for this heater by Rheem. (old ones shorter than part number called for for one thing). Anode looked lightly coated but not corroded at all. Too early to see if this ‘fixed’ anything since the rumbling etc was intermittent anyway, but I figure it’s in the “won’t hurt might help” zone.