If its as you pull away or move only, and its quite loud its probably the Manifold to exhaiust seal leaking, very common.
Good sympton is does it on pull away but not on just reving while stationary. Easy to replace the seal as well. You hear a lot of Mondeo's here in England doing it
Get underneath and remove the plastic covers in front of the Cat, remove the heat sheilding in the engine bay in front of the manifold. Back underneath to release the two bolt springs holding the manifold and cat pipe together. Release front most cat mounting rubber to allow some pipe movement.
From top of engine, push cat pipe down and put plastic bag over it to stop debris. The wire seal will be stuck on the manifold. Remove this with screwdriver, clean up and replace with new seal. You WILL NOT get old seal to re-seal as it compresses down.
Then reverse to get it all together. You can thread a little more of the bolts or pack a couple of extra washers on to get more compression on the seal, but generally its not needed.
I expect they twisted the joint a bit fitting the new exhaust and as the seal had compressed to seal at one angle and its now at a slightly different one, thats why its happening
Kits to do this cost $30 in England (Not from Ford) and include the seal, new springs, bolts and nuts and a replacement exhaust mount rubber.
Good luck
Mad Mole
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Mad Moles Mondeo Madness 2L Zetec, Cams, Stage 2 head, Valve work, Powerflow, Unichip, AEM Ram, K&N, Ecotek, 75mm Lower etc.
192 BHP, Faster than a speeding Duratec!
