There is loud knocking noise in my A/C pipes on and off.
I just had an A/C repairman out and he unclogged the line.

The knocking started back up within a day so the clogged line wasn't it.

I've been here 19 years and never had knocking in that area before.

The A/C repairman said that I have flex pipe and the building could have settled on it.

If that's so I don't know if the condo association needs to look into it.
I don't want to pay anything more to an A/C man but think someone out there must know what causes it and who can fix it.

PLEASE,PLEASE don`t call the same people people back,, you got ripped off big time ,whatever you paid them was too much,, He needs to be reported and his license revolked,people like that are what gives HVAC companies a bad name.

The only thing he could have unclogged would be the condensation drain line,and when that's stopped up there is no knocking sound
There is nothing in the copper freon lines that could stop up or that could get unclogged.
Also there is no such thing as flexable lines in your system (to much pressure for flexline)
The house settling on lines also 99% chance that is a lie also