Low pitched noise/resonance from drivetrain(?) - any ideas?

[Boston X5 4.4]

Hoping I can get help with a noise that I have noticed for some time. It seems to be worse now that weather is colder.

Its hard to describe but:

- The noise is low pitch/resonance. It is not high pitched/whistle like the noise others have traced to a hole in intake book, hoses etc.

- It is more noticable in lower gears under slow acceleration. It increases with acceleration and gets at its loudest in lower gears just before the auto changes up a gear at about 2,000 rpm. It does not seem to be noticable at higher rpms and higher gears.

- The noise/resonating when it happens seems to be coming from front and underneath.

Any thoughts?