Need help with an 88 turbo

I'm looking at buying an 88 turbo supra , but the owner said that while driving the turbo started to make a strange noise around 5000 rpms and he decieded to park it and throw it up for sale. I was wondering if anyone had any ideas of what this problem could be and if it is just the turbo, around how much would it cost to fix.

He is asking for $4000 obo with 88,000 miles on the car. The only mods done to the car are viper alarm, tinted windows and muffler.

I haven't heard the noise myself all this info was given over the phone. I realize that this question is kind of general. I'm knew to the supra line and just need some opinions.