1977 GE 6RS20SP9B9 - what the heck is this component?

I think it is a thyrector, a transient voltage suppresor.

Datasheet : http://docdro.id/eVE6OB5

why (noise)

It is a selenium rectifier.

Google search of the part number points to this.

http://www.parttarget.com/6130-00-136-1416_6130001361416_KZ9DTM.html/-1642D163-C569-435E-881F-72C91B685EF9

MRC     Parameter                             Characteristics
---------------------------------------------------------------
AARA    Terminal Quantity                     2
AARB    Terminal Type                         Wire lead
ABHP    Overall Length                        1.600 inches nominal
ADAV    Overall Diameter                      0.575 inches nominal
AJUR    Circuit Connection Style Designator   Half-wave 1 phase
ANPZ    Inclosure Feature                     Plastic case
ASCT    RMS Input Maximum Voltage Rating      270.0 volts
ASCZ    Stack Quantity                        1
ASDS    Grounded Terminal                     Not included
MATT    Material                              Selenium