Moaning sound from plumbing after flushing toilet....

Hello all, A while back our main drainage line was stopped up, so we had to have someone come out and snake the line from the outside. They fixed that problem but ever since, the plumbing has made some loud moaning sounds whenever the downstairs toilet is flushed. It can be heard from the upstairs bedrooms also. It's the loudest in the wall right behind the downstairs toilet and runs up the walls. It only lasts for a few seconds and then stops. I'm not sure where to start, so I'm hoping to get some advice here. Thanks in advance for your help! Tags: toilet, plumbing, pipes, noise, sounds, moaning