Honda 2007 Civic vibration

I have a 2007 Honda Civic with only 26,000 miles on it that recently sounds louder than normal when started up cold and when putting your foot on the gas until going about 20mph…then sounds normal again. dealer changed out the power steering column and water pump, which fixed that problem, but has caused a new one. Now the floorboard and under the drivers seat vibrates once you hit 35mph slightly. It feels like something underneath is vibrating. only a small amount in the steering column vibrates…dealer replaced the 2 right wheelbearings front/back but it still vibrates. the axle didnot need to be removed to install the steering column. dealer than changed out all 4 tires with new tires and it still vibrated…he put my original tires back on(also aligned the car and balanced the wheels prior to this)…dealer doesnt know what can cause this. all the bolts have been checked to see if they are tight. Is my car just a lemon? dealer is perplexed and I want my car to just drive smoothly like it did before they worked on it. any suggestions?

helpless in ct.

check your state’s lemon law before you try to invoke it. From what you describe, it could be a loose muffler pipe.