Poke "phase index mode" working patch?

It's hard to give advice without a simple example patcher and explanation of what you are doing + what you are trying to achieve.

But in general, if you are writing into a buffer that is being played back elsewhere, then anything other than writing at 1x speed (e.g. counter -> poke) is likely to introduce sonic artefacts. It's just the nature of how buffer writing works. Imagine the buffer as a tape loop of segments. If you write at >1x speed, then with each passing sample of real time, you may be skipping over some segments of the buffer. If you write at <1x speed, then with each passing sample of real time you can end up writing into the same segment more than once before moving on. Either way the data in the buffer is now a strangely mangled version of the input sound, which can indeed sound like ring modulation. My guess is that this is the problem you are facing.

The simplest solution to avoid this is to always write at 1x, and do any required speed changes at the reading end. Depending on what you are trying to do this might be enough, or you might be able to get closer to what you want by bouncing two loops/delays.

There are also ways to emulate writing at other speeds without causing these artefacts, but they are pretty complex and can have quite a bit of CPU impact, depending on how much you want to be able to change the speed. It's a generalized sample rate conversion issue.