Movement in the back suspension area
 

Hi guys I'm looking for abit of help ,I bought a 99 jdm WRX a few weeks back that had been laid up for 3 years .the problem is when I move off I can hear and feel a grinding noise in the back of the car ,there's new pads on the rear and one of the slider pins were stuck .it also passed the NCT with no problems at all . anybody got any ideas on what this could be ?

back plate corrosion?

I gave the back plates a good cleaning ,I did notice one of the slider pins stuck so I regreased it but I think it could be bent because it was getting stuck when pressed full in .....i may have to replace it it what you think ?