The disposal was moved from one drain to the other (left to right) of a double sink. It had been problem-free but just was on the wrong side for best cleanup. The work included getting bigger drain pipe as per code and replacing the filter on the nondisposal (left) side. When the glue for this work dried I noticed some new obnoxious noise from the left drain. Whenever water pours into either side of the sink the left drain makes noise.

When I told the plumber about the noise after several days he said he needed to check the air transfer gap. Would I need further expensive work or is this something that should have been taken care of under the original fee ($150 for 1/2 hour)?