When I do these as well it's excruciating in the left IT band, I also notice my left knee wobbles and caves and bows out and shakes like crazy when doing them, if that helps at all.
So a couple months back I noticed that my left IT band felt a bit tight when squatting, just not comfortable. I'd foam roll it at the end of workouts, and it'd go away. I checked form, stance, etc. and couldn't figure anything out. Over the last couple months it's gradually gotten worse.
The last month or so if I even do a squat with 135 I end up screaming it hurts so bad. My solution has been to foam roll the IT band for about 5 minutes before I even start warming up with the bar, and then for a couple minutes after each warmup set. This seems to fix the issue temporarily, and by my last two sets I don't need to foam roll, but next time I go to squat it comes right back.
In fact later in the day if I attempt pistol squats or something back home I actually fall over in excruciating pain from how bad it hurts. Foam rolling relieves the actual symptom, but not whatever the underlying cause is. The epicenter seems to be about halfway up the IT band itself. Any ideas? My right IT band doesn't give me any problems.