The wood pellets shouldn’t matter. My first guess would definitely be a leaky bag, but they’ve been holding air since the error message? Have you ever replaced them? If a bag is leaking then you will certainly hear the pump running more than usual. Can you let it sit for a couple days and then see if the rear end is sagging? normally the pump doesn’t run very much at all, just over uneven terrain, my sloped driveway approach, etc. Id still lean towards a very slow leak, and a leaky bag won’t fix itself. I’ve used Arnott replacement bags with very good luck (not a fan of their Jeep spring conversion though), and it’s an easy do-it-yourself job.

Beyond that the usual failure points are one of the self leveling switches on the rear subframe, by the air struts, corroded wiring/connector to these switches, or the pump/plumbing itself. There is an eBay guy who sells a rebuild kit to the pump, if it turns out to be that. New ones are expensive.

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Current
Original owner 2002 E53 X5 4.4i to 4.6i swap
2026 G80 M3 6 spd
2025 G06 X5 50e
Former
1972 Audi Fox
1986 Saab 900S
1996 BMW Z3
1998 BMW E36 M3 Sedan
2004 BMW E46 M3
2006 Audi A3 Quatro
1993 Mopar 318 Jeep Grand Cherokee
2015 V6 Jeep Grand Cherokee