"poppin" when i turn off mc275V

[mati1979]

Hi, when i turn off my MC275V , after 15 sec. I hear a low frequency noise ("pop") from a right woofer.

Anyone know what can it be ?

[mati1979]

concluded...
1 kt88

i replaced 1ers with 4rt and "pop" go to left channel.

1er slot kt88 is right or left channel ?

[ATC2]

Found schematic on line, I think 1&2 are left and 3&4 are right. But my knowledge on this is very basic so I my be incorrect. Take a look you may be able to figure it out.