I’ve got a set of slx brakes and have noticed the pads have started to chatter noticeably when going quick down hill I’ve checked to make sure it’s nothing else, the split pin that secures them is in place so is this normal?

Pad spring a bit loose? Try bending it out a bit.

have you regularly cleaned your disc brakes with a quality (bicycle specific) disc brake cleaner?

most noises and vibrations, or power loss, are caused by not cleaning disc brake rotors and pads, especially after washing the bike with aggressive cleaning products

once the bike is clean, give both faces (driveside and non-driveside)of each rotor a good spray

then spray the caliper bay (this blasts the pads and pistons) and wipe the rotors clean with a lint-free rag, you will be surprised how much oil or dirt comes onto the rag.

The trick then is to ride up and down your street doing increasingly aggressive emergency stops, until the horrible squealing noises vanish and suddenly you have very effective and quiet disc brakes 😉

Esher they chatter not squeal because they are able to move in the caliper.

@sturmey

remove the pads and pad spring

try gently bending the spring outwards

then re-install pads and spring

That dirt you wipe off is pad / disc material not grease / oil.

@bigyinn

its often grease / oil where spray lubricants have been used on forks stanchions and chains..and have drifted onto the rotors and contaminated the rotors and brake pads

or contaminants left by use of aggressive bike cleaning products (e.g. Muc Off)

in addition to a build up of disc pad material