I would ask if it has just started, If someone was tampering with the door.
Assuming that the conditions are not bad, then perhaps recommend that the cabin crew member put a wet cloth into the door seal where the noise is coming from.
I would then consider reducing our cruise altitude slightly perhaps 2000-4000'.
If conditions do not look good, I would still recommend a wet cloth in the area but without forcing the cloth into the area then reduce cruise altitude by 4000-6000'
I would get regular updates from the cabin and if the situation doesn't improve I'd consider an enroute diversion.
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