If it's not glow plugs it might as well be a small air leak into the lines, not that bad because the motor has never failed to start because of this.

(I could see air as a possibility with all the biodiesel i've been running...straight-up B100 for over a year now .. even though I had the lines replaced before I started using it)

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99 E300 TD -- sold
01 540i 6 spd