My thoughts on it, if the engine seems to be flexling and moving about slightly, then something may be amiss. A engine that is slightly loose( screws mounting it down, broken or cracked mount lugs), a cracked or broken mount, a firewall that is coming loose, mount to firewall screws and or blind nuts that are loose, or a firewall/fuselage that is flexing. I have seen where the blind nuts pulled into the wood and crushed it and ran up against the motor mount and wound up being loose in the wood although they were very tightly screwed down. The firewalls on some ARF's are a little on the thin side in my opinion and made from a softer lower quality wood too, plus the glue they use to gue it together is very minimal. I have reinforced the front end on several ARF planes so far because of that.

Of course if all that checks out, scratch your head and just go with it then, Happy flying.