power dip on gear change

[Darrenflounders]

When I change gear in my 57plt sprinter the power drops off for a second then picks up again as normal, it also sounds like the exhaust is blowing at low revs. Checked the exhaust and there's nothing wrong, no signs of damage?!?!? Got me scratching my heed

 

[Robbostanhill]

Sensor at the front under the edge of the front valance. Can't remember what it is for but that's what Merc changed when I complained of similar problems