The rattle is caused by the aluminum tube that covers the float and wires of your sender. The nut securing the tube has worked its way off and the tube is banging around in there. I know this because I've got one like it in my 300D. I pulled the sender and only came out with the internal float mechanism and some tangled wires. I put a replacement sender in there but haven't been able to fish out the tube. I figure that when I get bored I'll pull the tank and flush out all the algae and crud and get rid of that tube.

Rick

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80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)