"the I5 is inherently very smooth"

Really?! Then the I6 must be BEYOND smoothe. The I5 is inherently UNBALANCED, and requires a great deal of counterweights on the crank. Only I6's and V12's are inherently balanced, which is why they are prized for their smootheness, while I4's and I5's, which are inherently unbalanced, are used because of compactness, and efficiency. Trust me, our I5 engines are probably the smoothest I5's in the world, easily rivalling many V6's, and I4's, but to be honest, they aren't all that smooth.

And if you are getting a rumble, and it is a direct result of the bushing, then what is happening is the bushing is allowing more vibration to a part or area of the car, which is causing the rumbling sound. And this vibration probably isn't a good thing, if you can hear it.