Can't find source of vibration/rattle

[RECHRGD]

The last couple of times I drove the Charger last season I noticed a slight vibration after starting the car with the engine cold.  I could feel it in the steering wheel and the rhythm was in line with the shaking of the engine.  Once the engine warmed up and smoothed out it would go away except occasionally while stopped and in gear.  I'm now getting the car ready for some summer cruising and starting to try and find the problem.  I thought maybe a header pipe hitting the steering,  but all appears clear in that area.  Under hard acceleration there was no indication of a bad motor mount, so I didn't think that to be the problem.  Today I put it up on the lift and went under it while idling cold.  Although the engine was shaking as expected, I could see nothing touching or bumping anything.  Also today, I noticed that it felt stronger at the front of the car.  I could feel it really good on the metal in front of the radiator yoke.  Any ideas?  BTW, the drivers side mount has the two grade 8 bolts though it trick.