Weird guitar screeching/feedback noise?

just got my guitar back, a Fender Big Block Stratocaster, from getting it setup at a guitar shop: Long & McQuade. I plug it into any of my amps and when I put the volume knob on my guitar to 10 as I always have, I'm welcomed by this incredibly loud screeching sound. This also occurs when I switch pickups. The only way I stop it is by turning down the volume knob to 0. I only got this sound with this guitar after getting it setup; before it was setup it didn't do this. However, the odd thing was that the first time I tried this Strat, there wasn't this issue and I never had it before when I tried it out and prior to having it setup. The really weird thing is that when the guy plugged in another Big Block they had in stock, it had this same issue. This is just so confusing. It never occurred before with the one I tried out and the one I have, but now it's occurred in the one they had in stock as well as the one I have now. The tech said its cuz of the metal pickguard. I don't know