Adam, have you had any luck with the rattle?
Mine has developed a similar one (R400 Duratec). It seems to be coming from the passenger side somewhere, but I cannot pinpoint where exactly. The rattle, which sounds more like a loose nut bouncing around, happens in all forward gears at around 2500RPM to about 3500RPM (maybe more, but I can't hear it over the noise). I checked all of the components (by hand) and nothing is loose. The rattles is not there when the car is stationary (i.e. revving in neutral).
I did find the plug cover loose (all bolts were out, I have no idea how it still held in place 😳), but after fixing it back, the rattle is still there. It drives me nuts, to be honest. ☹️
On a side note, I have an "old couch" squeak from the passenger rear side as well on bumpy roads. I re-lubed the sway bar and it went away for a while, but it is back again. Is there a fix for it or do I have to grease the bushings after each track day. 😬
cheers