I have a Carrier Condenser heat pump that is low on R22 because of the high pressure valve was leaking. The compressor was making a terrible noise with this low pressure.

(question 1:) Is it normal for the compressor to make noise with low pressure? Or is this a sign the compressor is on it's last legs? (We did not leave it run with low pressure, it was fine one day and the next it was making noise so we shut it off.)

(question 2:) Shouldn't this system have a low pressure switch to turn it off when the freon is low? I do not have a schematic to see if it includes a low pressure switch but I was shocked to see that it is still trying to run on 25 psi.

You really should take a course in HVAC from all your posts and trouble you have with various units. You will be better served and then have the knowledge to do the repairs correctly.

It is not mandatory for manufactures to supply high and low pressure sensors for residential HVAC units.
If the compressor is making noise at low pressure it is a sure goner when the load increases especially on a scroll type instead of a piston drive/recip compressor.