100% of the time it will be exhaust. 90db is crazy quite and anyone with a non standard exhaust won't pass.
Then BOV and intake mods ie: exposed pod filter.
Then you are almost certainly going to be given an inspection notice. This is when things the police didn't find the DECC inspector will find.
If you use common sense you will know what he will and won't pick so be prepared before you go.
And no, high mount setups themselves are not defects.