Strange laptop hard drive behavior – Anyone seen this?

Have one in my shop that I have never seen before — Compaq Presario V4000 laptop – locks up intermittently – I have used all the usual (Hijackthis, Spybot, Adaware, Trendmicro Housecall) to clean and discovered that, if I “rock” the laptop by lifting and releasing its right, front corner, hard drive activity starts up and will go for an indeterminate amount of time. When hard drive activity light goes out, system seems again locked, I can rock it and it starts up. Would you say this is motherboard/IDE channel or hard drive problem?