Local loop currents used to be a lot higher than the modern loop. I have observed that many old transmitters other than the WECo brands, show a curiously high resistance, on the order of tens, even hundreds of kiloohms, when the current is low. Modern ohm meters use a very low current and make those transmitters look dead, but measuring them in a circuit constructed for the purpose using a higher voltage, say 10 V, source, the resistance measurement drops to more reasonable values.
Somehow this seems related to the noise problems, but I haven't studied that relation much. It's tedious actually. For some reason WECo understood the manufacturing of long-lasting transmitters better than anyone else.