I am converting my B-17 to electric. After running wire I ran the motors. They all run good except that there is a knock that happens every few seconds from the #3 and #4 motors/ESC . I can't tell from which the knock originates. Any Ideas? What about a capacitor?

TGY AerodriveXp 32 SK Series 35-48 900Kv / 770W

Model: SK35-48 900
Kv: 900rpm/v
Turns: 4
Resistance: 35mohm
Idle Current: 2.9A
Shaft: 5mm
Weight: 171g
Rated Power: 770w
ESC: 60A
Cell count: 3~5 Lipoly
Suggested Prop: 11*8, 13*6
Power equivalent : .25~42+ glow engine

Plush 60A Brushless Speed Controller

The plush series ESC are a very good quality controller. They have a broad range of programming features and a smooth throttle response compared to other BESCs in the same price range.
This BESC can handle 3A continuous current which is great for larger planes with bigger servos.

Cont Current: 60A
Burst Current: 80A
BEC Mode: Switching
BEC : 5v / 3A
Input: 5.6v-22v
Weight: 60g
Size: 80x31x14mm
Supported motor speed for all TURNIGY ESCs is;
(Maximum): 210000 RPM (2 poles), 70000 RPM (6 poles), 35000 RPM (12 poles).