Grease everything. Take the clamp apart, clean all the parts carefully and then grease every area where metal touches metal, clamps, captive nuts, under the bolt heads, on the threads and particularly around the saddle rails themselves. There’s always movement at the rear of the rails and movement means creaks. As mentioned above, make sure the clamp plates face the right way, there’s an arrow on each one that should face forward.
I thought the silver collar was for emergencies in case the seatpost failed – it enabled you to keep it at a fixed point. What are you using it for?
The silver collar you’re talking about is a travel limit collar for Enduro racing and the like. In other words its so you can drop the post just a bit easily and quickly and still be able to pedal without our knees being around your chin.
The other silver bit that might be referred to above is on the older Reverb models and is the top part of the external assembly. Checking that is tight would definitely help 😀