I've had an awful rotton bin type smell in my living room for the last few months. It appears to be coming from under the floorboards of our ground floor Victorian room. I removed the floor vent to have a look and put my hand in, and the sand type stuff (sorry I'm not very knowledgeable in the area) is wet.
No this is not normal, you have a small leak coming from somewhere, you may have to take a few more floorboards up and investigate, if water isnt coming through the ceiling then it may only be a slight leak coming from a radiator pipe!
I would take the floorboards up though before it gets worse.
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