Odor in clean claw space. About four of head space. Well vented. Lived here 15 years, home is 25 years old. No water. Not damp.
Three weeks ago we had a dead mouse in a duc. Replaced duc. Cleaned all of claw space. Spread Lime on vapor visquen barrier. Spead new black visqune. Set traps, nothing in traps. I thought it might be the smell from the new visqune, but it's been three weeks.
Ordor is not present from ducs when running funance fan. When running fan in craw space ordor is in parts of home. Smems like "dirty" dirt.
Any ideas?

I'd guess dead mouse, or other small animal or feces and urine in the ducts leading to the parts of the house that smell. The dirty dirt smell might be just the nest, which could be made of anything, insulation or straw or lint. Dead mice and excrement smell is absolutely awful and indescribable.

You can hire someone with a snake cam, or just hire someone to clean the ducts, and install screening or whatever will keep animals out (almost impossible). If the ducts aren't too long you might be able to jury rig something out of a shop vac and long hose.