Many of the sources of interference today didn't exist when antique radios were new. Switching power supplies in modern electronics come to mind as bad interference sources. On the other hand, if you read old manuals, they write about interference from brushes on motors of all sorts, electrical signs, streetcars, vacuum cleaners, and so on. Sometimes a line safety cap or two added to a set, or at the noise source, will help, if the noise is coming in over the power line.
http://www.justradios.com/safetytips.html