mcmaster carr has the quilted foam - someone on this board used it on his 240D and got some really good results.

You can also try to find some foam that can stick to the underside of the plastic engine cover - it really makes a difference, I know this as I have driven a Land Rover Discovery 300 TDI and that had a 2 inch thick foam pad on the engine to suppress noise. Removing that made it loud.

Infact Im also looking for a similar foam for the OM606 engine cover - it will dampen the already low sounding injection nozzles.

for a cheapie solution - look for a VW Golf mk2 diesel in the yards - it had a really hefty insulator pad on the hood which was held in by leaf spring clips about a foot long each. Its a really really good item.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)