hiya Vallon, U do not want loose blades in the grips thats a no no ,just like too tight is.. run them up with them fairly loose in the grips (not too loose though) then spool down without moving the blades and tighten the blade bolts. this is essential for later on when u begin autos and other stuff.. as loose blades will boomstrike.. believe me ,ive done it.. its not fun.
otheriwse forget about rock and shakes in spool up, its normal. as mentioned its very probably just the blades centering themselves. only worry if u have settled into a nice hover and the shake or wobble is still apparent. . otherwise just enjoy the feeling of knowing u r mastering one of the more difficult rc hobbies there is yeps too low a headpeed will make the heli shake, as will flybar off centre and out of balance along with main blades ,shot main shaft bearings and also the head dampers not lubed up enuff. and of course other reaosn too. also training gear will accentuate any vibes at all.. specially if its flexi fiberglass type. when i was at the traoning gear stage , i once had the heli shake so bad i had to land quick as it looked liek it was gonna explode.

Also Vallon, with the tail boom if u find the belt is continually coming loose drill i tiny pilot hole in from the top and put a tiny screw into the boom through the main frame to stop it loosening . i had to do that works a treat. just make sure u dun make the screw too long as to come anyweher near the belt.

laterss
james