While at speed and experiencing the problem, depress the clutch, observe, shift to neutral, observe. Did the problem change with either of these? The engine RPM will come down quickly, car speed will slow very gradually. Is the vibration/noise changing quickly with the RPM of the engine or staying at pretty much the same frequency/intensity?
With the car sitting in neutral, park brake on, open the hood and grab the engine, try to shake it, it should feel like you're grabbing any other part of the car and shaking it, if it feels very lose in the bay, you prabably have a mount problem.
the recalls: http://www.autobuyguide.com/2005/12-aut/cadillac/cts/recalls/index.html
I don't see anything that matches up there, though.
If the noise changed with the engine speed in the test above, it sounds to me like it could be an injector. Buy some good injector cleaner and see if it helps any. Don't put two or three different kinds in, it sounds like common sense but if it does help, run your tank low and refill before putting any other additives in (injector cleaner or other).
With the car sitting in neutral, park brake on, open the hood and grab the engine, try to shake it, it should feel like you're grabbing any other part of the car and shaking it, if it feels very lose in the bay, you prabably have a mount problem.
the recalls: http://www.autobuyguide.com/2005/12-aut/cadillac/cts/recalls/index.html
I don't see anything that matches up there, though.
If the noise changed with the engine speed in the test above, it sounds to me like it could be an injector. Buy some good injector cleaner and see if it helps any. Don't put two or three different kinds in, it sounds like common sense but if it does help, run your tank low and refill before putting any other additives in (injector cleaner or other).
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