During my time on ULAS as a student, solo spinning was still in the syllabus. So off I went one fine day to do the deed....
HASELLs diligently completed, close throttle, keep ball in middle, trim as instructed, then at the moment critique apply full pro-spin controls, up and over she went and into the spin.
After the first few seconds there was a loud thumping and banging from behind me, so I rapidly recovered - all was fine. A few gentle aeros - no problem.
I mentioned this when I came back - and the general consensus was that the knotted ends of the rear seat harness had perhaps been banging against the aluminium seat structure. So I went out to the aircraft (still 'soloed up') and experimented - yup, that was definitely the cause. Then a coffee, off for another trip including solo spinning (after first making sure that the rear seat straps were really tight and the ends tightly knotted together) - all was fine.
A few weeks later some edict came down from on high banning students from solo spinning. Rather a pity as it gave you lots of confidence!