The dealer came out on Thursday to replace the heater, but when he got the heater out and looked at it he said he didn't think the element vibrating against the inside of the heater tube was what was causing my sound, since the element was no where near the edges when he examined it. He went ahead and replaced it anyway since he'd already pulled it out and had a new one, but also did some nosing around inside the equipment area. He suspects the buzz was actually coming from two of the bare copper ground wires coming off the pumps that were zip tied together on their way to the ground bar. When I had done my initial hunting around for the sound I couldn't see behind the pack and so didn't see those grounds tied together. The tech cut the zip tie and rerouted the grounds so they don't touch anywhere. I just got home from a labor day weekend getaway and filled the tub - so far the sound now appears to be gone.