Frigidaire front load washer "05 thumps violently in spin cycle. Easy Fix?

I have a 2005 Frigidaire front load washer I inherited with the house I bought over a year ago. The warranty just ran out 5/05 too bad for me. Suddenly the drum makes a violently loud thudding noise when it spins out the clothes. We took the back off to get a look but don't know how to get inside the drum to see what the noise is. Should we procede ourselves or call a repairman? How do we get inside the drum? Since this washer seems to have cost about $700.00 is an expensive repair on an '05 reasonable? Thanks for any help. Model # WDZPPI-5 Liz