Which version is your PS2 ?
V0 to V7(or V8) have a metal tube which is used as guide. Grease from your finger may cause it to "jam" making it "jump" while moving in or out. And the weight of the tray on the metal tube causes it. The solution is dismantle it, wash all the parts with neutral soap (once you removed all the metalic parts on the tray, like the screw which holds the PS logo and the screw which hold the metalic tube.) then mount everything again in place.
If your PS2 is a 50k (V9 or V10) the tray is completely made of plastic and if you have grinding on that version you probably have broken one of the gears or made a dent on the tray itself...
SCPH-10000_GH-001 SCPH-15000_GH-003 SCPH-18000_GH-008 SCPH-30001_GH-005 SCPH-30000_GH-016(V4) SCPH-30001_GH-010(V4)
2xSCPH-10190, 2xSCPH-10350, 2xSCPH-10280
"**** j0 hackers!"
-Sjeep (As seen on TOXIC OS ELF...)