The battery has parasitic drains that will discharge it. The sound is a low volt alarm the smoke detector or co2 alarm puts out.
Charge the battery and disconnect the battery when stored to prevent it.
2011 351RLTS Eagle, Mor/Ryde suspension & pin box
06 F350 Lariat PSD, SRW, LB, CC, EGR delete kit, 16K DrawTite hitch, Timbrens, TST TPMS
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