Wah. The man wanted a simple answer to his query and he gets a rendition of Carmalita
hyeah:
Sir,
1. What you hearing is wideband RFI (hash) being generated by household appliances and wiring. In the evening the noise goes up because all those wonderful CFLs are lit up adding to the RFI.
2. If the noise is intermittent then it can be attributed to worn out pots, switches or even worn out tubes. You know what to do if it is so.
3. If the noise also has a grubby hum component to it then it time to change the main reservoir cap and the coupling cap between the output pentode and the triode. In 99% cases it will be ECL82/86 and both the section are in one tube.
Solution:
1. Move the radio to a less noisy part of your home. Switch on the radio, tune to a quite section in the MW band and rotate the radio 360deg slowly. You will find an angle where it is picking up least amount of noise.
2. String your aerial outdoors and use a coax to bring in the signal.
3. Most important. You need a very good ground. 80% of signal is lost otherwise. Ground will improve the S/N ration.
BTW Linda Ronstadt rocks. I hope she is still as beautiful as she appeared in the LP record cover back in the 70s.