+1 to checking subframe and engine mounts. I had a very similar squeak or creaking/cracking sound that I eventually traced to a broken upper engine steady bracket on the firewall. It holds the bottom end of the wishbone bolt, with the top held by the plate under the master cylinders (RH drive) or the heater hoses (LH drive). In my case the bracket had previously come off and had been brazed back on, but was loose again. In addition, the hole in the master cylinder plate had fatigued into a bigger hole, so the upper engine steady wasn't doing much. Engine torque from acceleration/decelleration translated through to the tail wing of the subframe where it bolts to the floor. There was a crescent shaped fatigue crack in the floor pan that made the noise.

There's supposed to be a strip of plastic between the U-bolts and the rack - I didn't think it would make any difference and left it out...
...yup, it squeaked.

You can get the proper strips, or I improvised and used some Visqueen damp-proof plastic membrane (the stuff that goes under a concrete slab) as it's about the same thickness.

No squeak.

Ball joints? Tie rod ends?

Jack it up and have a look

Can't think of anything rack specific that would cause that sort of noise.

The rack U bolts do have a think plastic isolator sleeve but can't see leaving these out would cause any difference once bots are secure.

As your model mini has a rubber mounted subframe I would suggest checking the rear mounts as these are in a similar position to rack, making any noise from them seem like it's from the rack. Also check the floor where mount bolts attach for cracks.

Are you sure that the noise is coming from the rack? Perhaps jack the front end with both tires off the ground and turn the wheels left and right. maybe you can isolate the noise.

About 10 years ago I replaced the rack in my 89

Ever since then I get quite a loud creeking...especially turning right.
Sounds like rack rubbing on body on the left side.
I have the u bolt nuts as tight as I dare tighten them.
What material is supposed to be between rack and body (ifanything)....or how else might I address this annoyance?