Toilet refilling periodically
I'm having some toilet issues. About a month ago, the plastic chain on the flapper valve broke, so I put in a new flapper with a nice metal chain. The flapper worked fine as far as I could tell, but then for some reason--maybe turning the water off and on to the toilet, lord knows that last time that was done in my rental--toilet started squealing about half an hour after every flush. Google research suggested the culprit was the fill valve, so I went ahead and replaced that too.
Now I have a new problem. Somehow water is leaking from the tank into the bowl--I can tell because there are mineral deposits down the side of the bowl, which never happened before--and the toilet will refill itself every so often. Sometimes it's only a few seconds, sometimes almost a minute long; usually about an hour or so apart. Searching on google tells me to replace the flapper valve, but I just did that, and there wasn't a leak before I replaced the fill valve. I didn't make any alterations to the flapper either. So what's going on?
Thanks for your help