Suntrust Bank Account Maintenance Fees Internet

I've held a mortgage with SunTrust for seven years, and when I took out the mortgage was given a free checking account, as a mortgage customer of SunTrust.

Now, I'm being charged a $20 monthly account maintenance fees during some months, because my account balance goes below a certain level in those months.  This level used to be almost my upper limit; as my income has grown so has my balance, so that now my regular balance is usually pretty healthy and only sometimes falls below a certain level.

The account was granted as a free checking account with no monthly fees, and never any mention of the possibility of fees due to a need to maintain a certain balance.  But as my average balance has grown, I believe SunTrust has seen an opportunity to take a little extra money from me, thinking $20 here and there won't hurt me that much, because of a newly imposed minimum balance requirement. 

The account is supposed to be free checking, no maintenance fees, and SunTrust has decided to change that rule and impose a fee when they want to.