Disconnect the wires to one side or the other. Then (assuming there is more than one speaker in the l/r set) disconnect the other speakers in the set until you find out which side set/individual speaker is the problem.
I'm guessing but I expect when you replaced the speakers you reused some hardware that is designed for one time use.
Often what is intended as sound dead-ing will become resonate to the lows if it's disturbed.