Intermittent loud engine at 3k RMP under load

Car: 1991 NA Miata / MX5 1.6L

I have a very strange problem where the engine volume triples between 2800-3100 RMP underload. However, this problem is intermittent. I can tell when the car starts if it’s going to be loud or quiet. It does not matter if the car just started or is warmed up, the loudness at that range will still be tripled.

Things that I have tried to remedy the problem:

• Check for exhaust leaks
• Try different exhaust systems (6 total configurations)
• Changed engine oil
• Changed transmission oil
• Changed diff oil
• Changed spark plugs
• Italian tune-up
• Replaced ripped roof
• Installed dynamat

Nothing seemed to remedy the problem