It sounds like you've corroded/dirtied/otherwise damaged the bearings of the BB. Stock cartridge BBs often have pretty badly sealed bearings, and rain can get into them fairly easily. It's probably nothing serious at this point, since there's no play in the BB. Your options are:

• Replace the BB (this will have to be done eventually)
• Ignore it (it may last for 1000s of miles in the present condition before getting worse)
• Try to force grease into the bearings by spraying a ton of grease in around the spindle of the BB, in the hope that you'll flush out the dirt or corrosion. I haven't used it, but I imagine the spray-can type of white grease would be good for this purpose.

I wouldn't worry about it too much, unless the sound drives you nuts. I have a bottom bracket that makes a noticeable tick whenever I mash too hard on it. I have a replacement waiting for whenever it gets worse, but in more than a year it hasn't changed at all...