Problem with speaker at higher volumes - answers????

My buddy who is "internet deficient" asked me to try and find a solution to a problem he has with one of his speakers (Wharfedale but don't have the model although it would appear to be a generic type problem and I assume not indegenious to that particular manufacturer.) He is experiencing what he says sounds like a "thwap" sound coming from one of his speakers when he is playing it loudly and only on very dynamic passages. At lower volumes all is well and apparently the amp isn't clipping. I defer this problem to those more expert than I who reside here... Any possible answers???