My RH450 doesn't make any whistles on power on or off. Also the casing is aluminum not steel.
The factory ships them in a pretty heavy duty box with the ends of the amp surrounded in thick foam. It'd take an amazing whack to dent it like that and the box most certainly would be damaged too.
How was yours boxed by the person that sold it to you? If it isn't pretty beat up, and you think it was packed well enough to not rattle all around during shipping, I'd be asking the seller a few questions.
For reference here's the factory box and packaging
