Clattery when cold
Join Date: Feb 2012
Location: United Kingdom
County: Greater London
It isn't necessary, although it does help keep road dirt out of the engine bay and it might conceivably prevent a bit of debris dislodging the auxiliary belt. Against their use, the soundproofing felt on the upper surface tends to retain water that encourages the subframe to rust.
The bolts need a Torx aka star bit. I don't remember the size, T30 I think.
The rear bolts often seize into the 'rivnut' captive nuts, which then spin uselessly in the subframe holes making them impossible to remove. That means the only way to remove the undertray is to wreck it.
You then have to cut, drill or grind away the rivnut's collar and then they can be pushed through into the hollow subframe structure and fished out. New rivnuts can be fitted using a special tool required to crush them into position (like Pop rivets).
Cable ties are a pretty standard solution.