Thank you for the tips list. Today I buyed a silicone spray, removed the fan and put a drop on the internal cylinder, reassembled all and put some batteries on charge. The fan sometimes run but without strange noises, so for now this is the cheapest solution working. In the future if I can, surely I would like to adopt a long lasting method to preserve the life of my battery charger.Welcome to the forum. Lots of info here
There are 6 basic ways to quiet the fan.
1 slap the Opus: may pop out the cells. so not very useful may pop cells out *
2 wd40/silicone spray helps sometimes :smells*
3 Place the Opus on top of a fan sucking the air from the bottom*
4 Replace the internal fan with a quieter fan.
5 replace the fan with external fan connected to the fan switch in the OPus: This is used most often*
6 use remote cell holders connected to the each of the internal cell holders*
* methods I have used.