I have the same thing happen to my car usually during colder weather or when I drive short distances for an extended period of time. For me it happens below 3000 rpm when I let go of the gas then I go back on really fast, It sounds almost like a rattle coming from the engine. My best bet is that it could be the valves but it doesn't sound like anything to worry about.
Bilstein B8 Euro-spec struts, Bilstein B8 shocks, eibach pro-kit springs, Remus exhaust, stage 3 intake manifold,Stage 2 AA tune, BMW Short shift kit. Silicon intake pipe. M3 control arms front, M3 rear. ECS trailing arm with whiteline bushings. Purple powerflex subframe bushings. Yellow power flex upper shock mounts. Single mass flywheel OE weight.