I think the Yuan are considered pretty quiet, but the Stirling AIP does produce some noise when it is operating underwater.
Plus there are the hull hydrodynamics and perhaps the internals aren't rafted because the Yuan is too small?
There's not much written on the topic of submarines anywhere.
Yuan is huge for an SSK. It is the second behind the Soryu-Taige class among the largest conventional submarines in service in terms of submerged displacement. Consider that the 056A corvette has rafted internals for nearly half of its displacement.