We used to shave the edges down on the lifters in our SRT-4's. The noise sounded like a diesel truck. It was a simple job for that motor. Not sure if that applies to the miata though.it's normal. if you REALLy want to fix it you can pull the lifters out then pull them apart and clean/rebuild them.. it's a very long process.
I would do a nice oil change before tackling that. and I correct myself from my other post. We filed down the cam caps to eliminate the noise they made. Wasn't thinking when I typed that.
I agree. I wouldn't take apart the head just yet..Get rid of the 20-50 oil and go back to a good 5-30 or 10-40..seafoam again..it might take a few short oil change intervals before they get clean enough to stop making noise.
the clearances on these things in that area are pretty tight so that thicker oil has a tough time getting in there..making the problem worse..go back to the 5-30 or a 0-30