Purring (or rumbling) rumblestrutting, short bursts of teeth chattering, humping, chasing, head butting, nipping are all part of dominance behaviour and acceptable. You have to brace yourself for quite a bit of that once Fred and Mervin are over the the first fright of being in a strange place. As long as Mervin sticks to loud squeaking, he's signalling his submission.
"Chinning" holding the faces together with the heads held up or prolonged intense teeth chattering with all hair standing on end is a warning sign for a serious fight! But that means that both parties are ready for the challenge.