Now they think it’s a sediment issue! That doesn’t make any sense either. Sediment makes noise, but not vibration. You’d hear snap, crackle, pop when the heater is firing. Of course, if that’s what you’re hearing, then it IS sediment.
I hadn’t thought of heat trap nipples, like Larry did, but that is a possibility. One of the types has a ball in the nipples that is prone to rattle sometimes and that can be broadcast all over the place by the pipes.
Maybe we should ask: does this occur all the time, or only when the heater is heating?