Somebody told me that leg-extension was bad for the knees. That is one on me. My knees are old, therefore, they are dam near worn out. However, I still do leg extensions foward and backward. Now if something starts hurting, that will be the cue to back off some.

You say you are hearing some crunching noises, IMO, I would let a doctor check that out. You don't want to abuse the knees to a point where they need surgery or stop working on you..

In the meantime, that was a good workout...