Do you have output errors too or any other errors under the physical interfaces in the portchannels

yes, 

ONATTDISTSW01#show int gigabitEthernet 2/0/42 | i Input queue:
Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 392676
ONATTDISTSW01#show int gigabitEthernet 2/0/42
GigabitEthernet2/0/42 is up, line protocol is up (connected)
Hardware is Gigabit Ethernet, address is 00f8.2c96.9a2a (bia 00f8.2c96.9a2a)
Description: Uplink to ONATTSW01
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:01, output never, output hang never
Last clearing of "show interface" counters 19:12:19
Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 392676
Queueing strategy: fifo
Output queue: 0/2000 (size/max)
5 minute input rate 1000000 bits/sec, 279 packets/sec
5 minute output rate 91000 bits/sec, 14 packets/sec
4184622 packets input, 1645110842 bytes, 0 no buffer
Received 75717 broadcasts (64568 multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 64568 multicast, 0 pause input
0 input packets with dribble condition detected
928998 packets output, 483778653 bytes, 0 underruns
392676 output errors, 0 collisions, 0 interface resets
0 unknown protocol drops
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 pause output
0 output buffer failures, 0 output buffers swapped out

What version of IOS are you on

   Model                SW Version           SW Image 

WS-C3850-48T       03.06.06E         cat3k_caa-universalk9

what do the physical interfaces look like for drops , are the drops on one or all physical interfaces

 the error is not balanced at the both interfaces

ONATTDISTSW01#show int po11 | i Input queue:

  Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 392676

ONATTDISTSW01#show int gigabitEthernet 1/0/42 | i Input queue:

  Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 0

ONATTDISTSW01#show int gigabitEthernet 2/0/42 | i Input queue:

  Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 392676

try change the load balance so it starts using both inks , we use src-dst-port works well but you should have multiple options , first look I would think your over utilizing one side of the po causing it to get hammered resulting in output drops , if you balance the traffic out it may slow down the drops

also as Leo sad are you using qos , it may be worth setting it on the switch , is it a user access switch

Load Balance options

(config)#port-channel load-balance ?
  dst-ip                 Dst IP Addr
  dst-mac                Dst Mac Addr
  dst-mixed-ip-port      Dst IP Addr and TCP/UDP Port
  dst-port               Dst TCP/UDP Port
  extended               Extended Load Balance Methods
  src-dst-ip             Src XOR Dst IP Addr
  src-dst-mac            Src XOR Dst Mac Addr
  src-dst-mixed-ip-port  Src XOR Dst IP Addr and TCP/UDP Port
  src-dst-port           Src XOR Dst TCP/UDP Port
  src-ip                 Src IP Addr
  src-mac                Src Mac Addr
  src-mixed-ip-port      Src IP Addr and TCP/UDP Port
  src-port               Src TCP/UDP Port

theres also a similar bug if the output and total counters are incrementing together but looking at your last post there not but maybe something to keep an eye on ,I don't see 3.6.6 in it though

https://bst.cloudapps.cisco.com/bugsearch/bug/CSCvb65304/?referring_site=bugquickviewclick