Relay type noise with KPA500 and PR6

[K9IR]

I am hearing a rapid relay-type rattling noise from my K3 whenever I transmit over 30W barefoot and the PR6 with the ACC option is connected. This wasn't happening before I added a KPA500 into the mix. I don't hear the noise when power is below 30W barefoot, nor when the KPA500 is inline.
 
I am using the KPA500 AUX cable, and the Elecraft Y cable from the K3 so I can connect both the ACC option for the PR6 and the KPA500 to the K3 ACC port.
 
I have searched the reflector for insights on this behavior and why it is happening, but not sure anything I've found applies--expept the possibility of needing to remove pin 10 from the KPA500 AUX cable--?
 
Guidance, wisdom and other assists are most appreciated. Thanks.
 
73 Paula k9ir
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[Mike K2MK]

Hi Paula,

Do you have a separate receive only antenna connected to the KXV3A RX IN BNC connector. If yes, then you are possibly hearing the COR (carrier operated relay) protecting the receiver front end from overload. Disconnect the antenna connector and see if the noise stops.

73,
Mike K2MK

K9IR wrote

I am hearing a rapid relay-type rattling noise from my K3 whenever I transmit over 30W barefoot and the PR6 with the ACC option is connected. This wasn't happening before I added a KPA500 into the mix. I don't hear the noise when power is below 30W barefoot, nor when the KPA500 is inline.
 
I am using the KPA500 AUX cable, and the Elecraft Y cable from the K3 so I can connect both the ACC option for the PR6 and the KPA500 to the K3 ACC port.
 
I have searched the reflector for insights on this behavior and why it is happening, but not sure anything I've found applies--expept the possibility of needing to remove pin 10 from the KPA500 AUX cable--?
 
Guidance, wisdom and other assists are most appreciated. Thanks.
 
73 Paula k9ir

[K9IR]

Many thanks to all who replied to my post. The issue appears to be "the Pin 11 problem"--which I'd read about before posting but wasn't sure it described my situation. Emails from several of you have helped me understand this issue better and I believe a modified cable with Pin 11 removed will resolve the issues I've been experiencing when both the KPA500 and the PR6 are plugged into the K3 AUX port using the Y cable.

I really appreciate the time so many took to respond to my query. Building the amp was very straightforward and it is working like a champ!

73 Paula k9ir