Thumping Sound When Flushing Toilet

[Bohowanderlust]

Last night we started hearing a thumping/hammering/vibrating sound whenever we flush the toilet, sound can not be reproduced when using faucets.

I've searched here on the forum but didn't find any suggestions. I Googled it and found similar complaints regarding the "Water Pump", but we are not using the water pump. We are hooked up to "City Water" at a State Park, where we've been before but not at this same site. We've called our dealer's service dept since they replaced our whole toilet about a month ago, but as always they seem useless in helping us, saying they've never heard of such complaint and they are overbooked to get us in anytime soon.

What we've tried so far, though we tried all of this the noise is still happening:

~ Read about "Hammering" pipes and water flow. It was recommended using a Water Pressure Regulator. We already have one in place.

~ Also was recommended to use two Water Pressure Regulator, one on spigot end and one on rig.

~ Dealer suggested taking the Water Pressure Regulator completely off.

~ Dealer suggested turning the spigot water flow down...tried 1/2 way on then tried only a 1/4 way on.

~ Opened side compartment to look at the pipes behind City Water inlet...nothing is vibrating there. Sound seems to be coming from deep within the belling of the beast. LOL

Help! Any suggestions? We are concerned about pipes busting etc.

Thanks in advance!