My 525i with M50 engine rattles on start up, but this noise goes after about 30 seconds.
My local BMw dealer reckons it might be the Hydraulic valve lifters making the noise before enough oil gets to them.
I have had the oil changed which reduced the noise a bit, but hasent eliminated it entirely.
Its worse when it's a cold morning.
Anyone heard of this, or no anything about these valve lifters and if they are replaceable?