Problem with Maytag PAVT234AWW washer?

My washer is less than 3 years old. stopped in mid spin cycle this weekend with a loud noise, and would not restart. Technician (not from maytag) came out and replaced pump b/c bearings were shot and belt. Cost $200
However, now it is making a very loud knocking noise throughout every cycle. He says I need a new tub and it would not be worth fixing b/c it would cost 300-400 and I've already paid 200. It was doing this sometimes before, but never this bad.
I called Maytag and they said tub is under lifetime warranty. Why would the tech not have told me this -- does it have to be a Maytag technician to get the warranty?
Does this sound plausible that the tub could be causing this much trouble? The guy didn't sound confident but said it had to be the tub.

Heather Crews
January 2007