what i would do since you said it had alot of old build up coolant is : do a flush and when you put the new coolant put a bottle of stop leak since you can't see the leak and for the knock or tap in the engine when you do an oil change don't use 5-30 or 10-30 oil use something like 10-40w and a quat of lucas oil. if that knock gets worsed thats not good your gonna have to get an engine hopefully that won't happen