Commenting to bump this up because I’m very curious to know what’s going on with your chimney, too!

This looks like water. The likely sources are:

1. rain

2. condensation

If you can honestly rule out rain (for sure sealed at top), not getting through brick, rain cap on top - then it has to be condensation.

An inspection camera run up through the metal flue will probably tell you what you need to know. The camera can be purchased online for about $25 that connects to your laptop or tablet.

Here's a tip:

It can often be very worth it to ditch the naturally vented gas appliances when you have that opportunity. They have terrible efficiency - about 62%, In addition, they are sucking warm air out of your house and sending it up the flue - with makeup air coming in around the windows and doors. Lastly, maintaining a chimney like that is a problem.

Often for the additional cost of lining the flue you could have switched to a sidewall vented appliance that gets air from the outside.

Thanks for the responses!

The only reason I was thinking of a clean out was to see what is on the inside of the chimney near the bottom, and to create some venting. Maybe not safe? Carbon monoxide?