Try blocking off the various sink holes / drains one at a time or all to begin with. Plugs in where possible but given it's such a quiet noise, shouldn't take much to muffle it if that's where it's coming from.

All the drains should have water traps on them but perhaps it's coming through on the pipe itself?
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Christian
#40104192 120Gb (no longer in my E36 M3, won't fit the E46 M3)