e46 M3 question: What's this box behind the exhaust for?

[MPETT]

Anyone know what this plastic cover hides beneath it?

Having looked from above there isn't anything obviously under there. There is a small hatch though!?

[gaz1234]

no

[chris116]

Diff cooler perhaps?

[E30M3SE]

It's the cover for the activate charcoal filter, the diff cooling is handled by the fins visible in the above picture built in the diff's rear cover.

[Pig Skill]

Yep charcoal filter cover - you could remove it to have a look but the bolts will be rusty as hell