The piston to head clearance is very tight on these engines. I have seen many a "lifter noise" turn out to be carbon buildup on the piston causing the piston to actually touch the cylinder head. The fact that you say "never been beat on" makes me think this is probably the case here.You can do a decarbon but it has to be done in a very particular way to be effective, and might have to be done more than once.