Does the 320i have an engine cover?
I have seen this engine cover issue with a Mercedes Diesel, the cover came a little loose and the diesel rattle was more pronounced, when it was put back on properly, the engine sounded much better.
'05 E90 325i 6sp Man SG 118,032km on the odo (retired) | '06 E90 325i Step SG 121,021km on the odo (retired)
'08 E90 325i LCI Step SG 130,024km on the odo (retired) | '13 F30 328i M-Sport Step 49,709km on the odd (retired)
'14 F10 535d M-Sport Step 50,179 on the odo