It is the lifters making the noise. If you've torn down these engines you would understand why. It's not a big deal, the worst it can do really is wear down some cam lobes a little quicker than normal. Running a thicker oil will make it quieter, but that's not necessarily recommended. I'm almost positive the what made it quieter after the seafoam treatment was just that it dislodged some of the sludge/carbon/gunk and ended up thickening the oil. Generally you're not supposed to run for more than 500 mi on the same oil after seafoam because of this reason. Different oils and filter designs (specifically the way they are valved) will result in an amplification or reduction in the noise. Just use a good oil like mobile 1 or royal purple and a quality filter and don't sweat it. This is not an inexpensive fix, you would have to tear apart the motor, and you could end up with the same noise in short order. If you're dead set on fixing it just drop in a motor from inertia motorsports or something. Honestly it'll be the least frustrating solution.