Greetings,

I am encountering a very weird noise on my 2009 Cooper S with the N14 (manual trans). When lightly accelerating around the 2000-3000 RPM range at lower speeds (in 1st and 2nd gear) as well as light acceleration in the higher ranges (3000-4000) in 3rd and 4th gear, I can hear an audible rattling noise coming from the engine bay. It sounds quite similar to the sound a woodpecker makes, but instead it's coming from my engine bay. The noise is reproduceable in any gear, as well as neutral while the vehicle is sitting completely stationary. The noise dissipates when I increase the amount of throttle I give the car.
The car is running completely fine, no problems making boost, normal temps, functions completely normal other than this stupid noise...

I've done quite an extensive amount of motor work to the vehicle, meaning I rebuilt it entirely replacing everything except for the pistons, connecting rods, crank, and cams, rod caps, cam caps (in the head). I put in all new valves and valve seals, main and crank bearings (from King), head bolts, head gasket, rockers, main bolts, water pipe & gasket, braided stainless turbo oil feed line, exhaust manifold gasket. I did the timing job as well using torque specs from the Hanes service manual.
The rattle started after I swapped this motor into my car, and I thought it may have been something related to the turbo, so I replaced it; No fix. I have an upgraded front mount intercooler and a catless downpipe as well, but I do not suspect they have anything to do with it. Also swapped the trans and gave it a new clutch while it was all apart.

I've done a little reading and can't seem to find anyone with the same issue.

Could it be a heat shield? Something in the intake manifold that is acting up? A bad spring?

I don't plan on taking it to a dealership (aka stealership) and will pull the motor for the 3rd time to fix it, so if you have any ideas or have encountered this issue and have a fix or potential ideas, please share.

 

Have you tried to locate the noise source using a long screwdriver or wooden stick?

Since you have worked on the engine yourself and have searched on the net for similar noises, I doubt what happened to my Mini is relevant, but just in case you missed it, here is a link to it:

Edit:
It would be helpful if you posted a recording of the noise.

 

Check out your belt tensioner damper (small black hydraulic shock). Over time the plastic bushings at either end dry out, crack and disintegrate causing the damper to rattle against the attachment bolts at either end.