I would just reflow all of the solder joints. The common one that we see a lot is the junction of the black wires on the headphone jack, where one will be well soldered and the other will be completely loose. I'm not saying 100% that this is what happened in your case, but it's sufficiently possible to rationalize at least going through and reheating all your joints to be sure.
As to why this happens at specific frequencies, that would depend a bit on what's actually going on with the driver. If you take a DVM and set it to measure DC resistance, what is the DC resistance between the sleeve and the tip vs. the ring and the tip on the headphone plug?
It's also possible that there's a hair stuck and rattling against the headphone driver.