With Cisco NX-OS the value you specify with the packet-size option is the payload size. This is different to IOS where the value is the packet size.
When you send the pings from the Nexus, you're specifying 1500-bytes which with the ICMP and IP headers, results in a packet size of 1528-bytes. Unless you have jumbo frames enabled these packets are then being fragmented before being sent.
Can you try ping <3750-ip-address> count 1000 packet-size 1472 df-bit and see if you have the same issue?
I don't know any commands that can be used to positively confirm this, but I suspect the small loss you're now seeing is due to the additional effort for fragmentation and re-assembly of the large packets.
You could check the output of show process cpu | ex 0.0 to check the CPU utilisation of the Catalyst switch when the Nexus is pinging it, and maybe the show ip traffic to see if any of those counters increase when there's a ping drop, but I don't think either command will give you any definite results.
Given the hosts attached to the downstream Layer-2 switches should not send traffic that will require fragmentation, I think it's probably safe to consider the link and network connectivity between them as "clean".
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