1) Horizontal Resistor contribution
It is placed at the input so it's noise appears directly:
Vn1 = 4kTR (R=300Ω)
2) Vertical Resistor
I usually calculate the output noise using equivalent voltage/current, and convert to the input. Finally I put it square.
Consider the noise as a current source I. It sees 2 parallel resistors, so the output voltage is I*R/2. The gain is 1/2 therefore the input equivalent voltage is I*R.
Taking square for the noise I get: I*R², where I=4kT/R. Therefore the input noise is
Vn2 = 4kTR
3) Total
Vn_in = 2*4kTR
I think is you do a noise simulation you'll get a result that matches.
Remark about 2). For noise the input is grounded. So your circuit is symetric... Therefore it is easy to see that both resistor will have the same noise contribution. Sometimes you don't need to do all the calculation