Help if you can...

I have an 03`Z, 18k, babied, nearly mint. Synthetic 5w-30 since 7k. No abuse in the form of hard driving on a cold motor, infrequent oil changes, etc. No dropping/slipping the clutch for fast launches. 93 octane with no ethanol crap.

My car ``knocks/clatters`` some under acceleration. It happens more in this horrible cold (20F). It happens only under load, revving in neutral sounds perfect. Above 2000 to...4500 (where it gets too loud to hear it?), I have a very rhythmic vibration from under the hood. Sounds like a hard plastic or possibly metal, though not like metal on metal. The best description I have for it is when you roll your tongue to do Spanish words ```rrrrrrrr``. Maybe 5-10 beats a sec.

I have a UR flywheel in there, but I know what it sounds like: cement mixer ``whoosh`` at moderate to heavy throttle , sometimes a clatter. Never this rythmic. So, what the hell is this?

It`s probably been around for the last 500 miles at least, but only in small doses. When the car was warm in above-freezing weather I didn`t hear it much at all. The cold seems to bring it out loud and clear.

The car drives great. Doesn`t feel like timing is being pulled.

??? Injector???