Ok, so ive had my first miata for about 6 months. But the other day, i started noticing a strange noise. At about 4k-5k rms, i hear this knocking noise that sounds like its comes from everywhere. I can feel it in the shifter, the gas peddle, and the floorboard. At first it was just a few knocks here and there, then it would stop for a bit, come back, go away, so on and so forth. But now, after i get on the interstate and cruise around said rpm's, it starts. A lot. Sounds like someone rolling an "r" sound when speaking Spanish. It doesn't do it when im gassing it, only at cruise and backing off throttle. If i pop out of gear or press in clutch, it stops. But if i rev iy back up it does it again at the same rpm's, even if not in gear and still rolling.

Car has a VERY bad clutch, and is getting replaced soon in a few days. Found out that i had a bad pcv valve, so i replaced it today, and that solved the oil consumption issue i was having . But i still have an issue with that noise.

Also when i turn my car on, every so often it turns over, then just dies. Turn it on again, and its fine. Sometimes i pull up to a stop and my rpm's dip, bob all over the place, then it either regulates or bogs out and dies. Ive ran codes, said a misfire or o2 sensor. Now, ive ensured ive got spark and all of my injectors ohm out properly. And ive never heard of a bad o2 sensor causing all of this.

When im cruising around town, car feels great. Responsive, solid. It only bogs when at idle, and very seldom. And even when its making that noise and im out on the interstate, same thing. Feels responsive, drives normal, no speratic rpm's, everything feels solid, then that noise/feeling hits. But overall feel of the cars drivability feels normal.

Has anyone else ever had this or know what it could possibly be? Please help. Any input is appreciated