Unfortunately the fans need to run so fast to cool the chip. Decreasing the fans speed would roughly equally decrease the graph/hash rate of your G1.
The only solutions would be to replace with low nois fans, or change thermasl completely, e.g. by repositioning the heat-sync. See this post, scroll down to see the post on modified thermals:
I have not done this myself and it might be a bit risky, so the easier solution would be just to replace the fans with some low noise fans, e.g. these 12 mm low noise scythe fans maybe:
Probably there are better people to answer this question, e.g. if you past on the blog of the first link I gave you, these are people who actually did modding on the G1 Mini and know best.