I have a Trane XL 90 furnace. I just noticed there is some water under the furnace. What can this be?

I had this problem, and it could be that the drain system is clogged *thats what was wrong with mine* or your filter needs changed.

I hope an expert comes a long for you soon.

But until then, you can at least check the filter.

I agree with jennie, furnaces have a drip pan inside where condensation collects. Drip pans have drain wholes and tubing, both of which can clog. Burning gas and propane always produces moisture and it must get redirected