A little knocking!!!!knocking scares me!!!!?

I have a 2001 ford zx2 101,000 miles,the needle tends to wonder to the hot side,and the antifreeze will boil a little from time to time I can actually see it bubble,and the last couple of days I have been hearing a faint knocking sound that goes in and out randomly when I accelerate it does not do it all the time but it worries me,and the car has such a rough idle, I just got a oil change they said i was a quart under,I got a coolant flush,radiator cap replaced.its still knocking the last time my engine did that it got worst and I had to have my engine replaced (another car)some said fill it with high octane gas,really its only a ford,plz mechanic or anyone that knows about cars!!!!