Dropped packets in portchannel without bandwidth issue

Hello experts,

I'm seeing the following: 2 Cisco MDS switches connected over a portchannel which consists of 2 ISLs. Each ISL is running at 4Gb, so the portchannel is 8Gb. We're seeing lots of dropped packets during 11PM and 6AM and II'd like to know why this happens. The physical layer seems ok, because during the day we don't see any errors on the ports at all.

Somebody says it could be because some storage cannot process data fast enough and the data flow has to slow down because of that, but in that case I expect to see dropped packets on the ports the storage is connected to, not some ISLs!

Does anyone know why this happens?