'Clicking sound from bulkhead' ... I've kept my ear to the ground on this one and it seems this clicking or grinding sound from the bulkhead (slight to the left from driving position ) also causing poor running. Is the motorised throttle body (butterfly valve) not then infamous swirl flap operation.
This fault will bring up code P2111 (motorised throttle stuck open) basically the motor/circuit fails... this will not illuminate the engine management light giving little clue to fault, but will show up via ECU scan as either stored or fatal.
SYMPTOMS: Holds a whole bunch of symptoms depending in level of failure including, hard to start engine, hesitation, misfiring, running rich or lean and erratic idle, black smoke under load. Could be mistaken for an EGR fault !!
CHECKING: easily checked via ECU scan for fault code P2111, remove inlet pipe and while revving engine assess the butterfly operation, run a actuator check on ECU scan, or via ECU scan check throttle opening position which usually remains at a static 5% even while revving.
REPAIR: The magneti mirelli unit is part no 48CPD4 and can be sourced second hand easily from alfa jtdm, vectra/Astra cdti, saab 9-3 as cheap as £30 from ebay or brand new around £150+ .. Dealerships price more than likely overpriced and expensive. Easy to replace in 30 minutes, you have two size 10mm bolts holding On the intake pipe and 3 Allen key holding the body to the manifold, electrical connector underneath body, new gasket required !!