Sounds like you have detonation or pre-ignition. What octane of gas are you using? You may have to step up a grade depending on temperature.. im sure Texas can get pretty warm, so you may have to use a better fuel.
Also the muffler shouldn't leak water as the exhaust system doesn't contain water, just exhaust gas. Did the techs try to sell you anything when you took it in? That's usually a hint they're full of it.