Sight unseen, sounds like your dampers need to be adjusted. They may not be seated correctly. The damper wires can be bent by a technician or it could be as simple adjusting the sustain pedal. Some of your damper felts may be brittle or worn out. Take the lid off and have a look. If a damper is not seated right, you can just gently lift if and set it back in place. If this is a vertical, you can play around with the nut on the pedal. Its fun to experiment. But if the damper wires need to be regulated, don't do it yourself. You may wind up breaking something.