Just installed a new stainless manifold, down pipe and decat on my 202. However it sounds abit sh1t between 2500. It sounds tinny/raspy. Has anybody got any thoughts as to what might be the cause?

exhaust leak?

The header itself and the removal of the cat. What you can do to help is install a small Magnaflow Magnapack(glasspack) past the downpipe to help cancel out any harsh unwanted frequencies. Like Bitter mentioned double check for leaks, particularly at the mating surfaces as the gaskets they include don't tend to be the best. Another option to help would be to use a chambered muffler to further cancel out harsh frequencies.

Do you have an after market muffler or is it stock?
Is there a resonator in the exhaust still?
Sometimes itll sound brutal without a cat or resonator.

Yeah double check there's no leak. I had my exhaust done a few months ago and it sounded raspy and I couldn't figure out why, took it back to the shop a couple months later and turned out it was leaking from the O2 sensor that they forgot to plug in lol.