The passenger side engine mount cops a beating in the TD42, as the hefty torque is trying to rip that engine mount apart, so I would try to keep the heat shield in if possible, or change the engine mount out for a stronger aftermarket version.....
Most people with the TD42 have had to change out that mount at some stage if they have kept their truck for a little while
My old GQ had a vibration like you describe, and mine ended up being part of the front drivers side A pillar metalwork bracing being cracked ( just like the inner guards do too ), and the harmonic vibration was causing it to vibrate at low speeds.
I only found it by taking the guard off and looking, as the noise was driving me nuts .. I looked for months till I decided to dismantle, but i was fairly sure of the region the noise was eminating from.