All good advice above but I'll add some to mentioning the trap syphoning because of ALL THAT's WRONG with the installation, especially it being an S-trap and/or not being vented properly. One purpose of a vent, aside from allowing water to drain properly and not syphon out below the trap-weir, is to allow sewer gas to rise up and out above the roof where it dissipates . Even a 1/16 of an inch will allow sewer gas to escape and cause that wunnerful aroma you're experiencing. // Sewer gas is noxious warm air and will rise at first, then, once it cools it will "settle" downward because it's heavier than air. This is why you "may" be smelling the gas under your sink ... though there may be other reasons. // Think of one purpose - there are many - of proper venting like this: Put a straw in a glass of water. Put your thumb over the top and raise the straw. Water will stay in, but will flow when thumb is lifted. // I know folks hate to call professional plumbers, but oftentimes your health, safety, and comfort are more important than saving a few bucks. Any "good" plumber has years of training and is as important as any doctor, nurse, or other health "professional". // Good luck.