Odd noise coming from tub drain
Several months ago I started a remodel of my guest bathroom and wanted to flip flop the tub so that the plumbing was on the opposite wall.
Originally the plumbing for the tub/shower was shared with the plumbing for the washer (the laundry was in the adjacent room and the wall inbetween housed the plumbing for both).
When the contractor flipped the new tub to the opposite wall he "hooked" the plumbing up (I know that is not the correct terminology) with the line for the sink.
Recently, I have noticed an odd noise coming from the tub drain - sort of an echo-y sound, as if you can hear the water/air from the pipes reverberating out of the tub drain. It happens at random times and I have not been able to "make it happen" by doing something such as running the sink faucet. However, it seems to happen on occasion, shortly after I have had water running from another source.
I am wondering if the contractor either a) should not have flipped the tub and hooked up to this other line, or b) forgot to install something, or installed something incorrectly.
The sub that the contractor had doing the job was sort of a jack-of-all-trades, and not a plumber per se. I am wondering if I should call a plumber in for an expert opinion, or call the city inspector back out.
Any suggestions? I have been in my place for 20 yrs and have never heard this noise before.