hi bro, im doing gym as well but not pro.
1.some machine that makes noices may due to lack of maitainence or the machine is too old.
2.when ppl doing it fast, normally they are using heavy weight or they are aiming for shape / cutting, if you want to gain strength, u have to do it slow
4.maybe ur fren mean exercise other parts except the injured parts?
5.some say solid food not enough coz not all we eat can absorb by our body, supplement may help
6.its normal, try doing ab on floor also, i mean besides using the machine