Agreed, sounds like air sneaking into the lines somewhere between the pump and tank also experienced on ageing delivery system TD42’s & 6.5 shed queens.
Talcum powder (puff puff - hydroscopic substance) is an old trick to pinpoint moisture leaks.
A diesel line pin hole/loose clamp/ filter seal may only become evident after the system is shut down at night…..back flowing, off forwards pressure if that makes sense.


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