Odor around air compressor

This sounds a little strange, but I have noticed an odor in the vicinity of my air compressor that smells like mercaptan, the odorizing chemical that they put in natural gas. There are no gas lines in the vicinity. I haven't always noticed it, i.e. not when the compressor was new. But it has become prevalent in the last year. This is a 6 HP single stage compressor with a 60 gallon receiver; probably 8 years old (+/-). Could this be a by-product of oxidation (rust) inside the receiver? Baffled!!