This is how mine is fitted. I have a couple of large, thick washers behind the bottom bolts.

It's one of those modern plastic coil sprung things. A car of some sort. Probably got those fancy wind up windows and such like.

Sorry, should have mentioned it is a series 3.

Yesterday evening my heater matrix gave up the ghost and dumped most of the coolant into the passenger footwell. After bypassing the heater and refilling the radiator, everything seems fine with no coolant loss so thankfully no head gasket...

I’ve gone with steel for now since the other ones seem to be! But saying that the other ones don’t have spring washers on them haha, Putting the nut on has stopped the firebell noise and as a second bonus, made the engine wayyyyyy more quiet!

It sounds to me like the plate that the bolts that go into the exhaust manifold go through is loose and rattling against the exhaust, and the exhaust is also rattling against the manifold.

I'm going to say it could be exhaust related, check all the unions