If tapping on the pickup fixed it at all, then I would suspect the pickup isn't getting a good enough seat in the saddle. Unfortunately, the fix would be somewhat technical. You might have enough slack to remove the pickup just to check out what the saddle seat looks like without the pickup in it, but I doubt it. It will probably need to be unsoldered from the connector to be able to see under it. If you don't have a problem doing that, you could give it a go. If not, it's definitely a job for a technician. I believe the seating is somewhat critical because that's the ground connection for the pickup (it only has one wire...the "hot" one) and it needs to be solid. Normally, the string pressure on it would make a good connection, but if there's something keeping it from getting a good connection, it can wreak havoc.