Either the plumbing is not vented properly or there is an issue with the p-trap. Most bath tubs have an access panel in the room on the other side of the tub/shower wall. If you do not have one, you can easily create one like I did.
Have someone in the bathroom knock on the tub side of the wall right where the faucet is. This will identify the cavity (between the framing) where the plumbing is located. Cut a straight horizontal line in the sheetrock until the knife reaches the framing. With the knife, follow the framing, vertically down to almost the baseboard, cross over with another horizontal line to the opposite framing, up and over to the beginning cut. Save this piece of sheetrock. Frame it like a picture, paint it the wall color...this is now your new access panel to get to your tub/shower plumbing. If you do this yourself or instruct someone to do it properly, then you do not have to do any costly wall repairs whatsoever. To secure the access panel, just use two screws, one on the left portion of the frame and one on the right, which will screw directly into the wall framing. Plumbers are plumbers, not sheet rock guys, and will most likely leave you with one helluva messy wall repair job, if you allow them to. Take control.