8 ft. is the standard, but I have seen contractors cut them off because they couldn’t get them in the ground properly. I’d double check it!
If all else fails, the power company will probably send someone out with some test equipment at no charge. If you end up suspecting interference, the FCC will send out an investigator too.
You may be picking up EMI interference from the incoming AC. I can tell when my nieghbors furnace is running as my incoming HV power tranformer is downstream from my nieghbors. I agree with Bruce. You should have it investigated by the utility first. Your ground should be 25 ohms or less. You could have a high resistance ground, improper bonding, or a problem with the power co transformer. Good Luck.
Have you, or a near- by neighbor, recently put a new Plasma TV into service? Some of those devices are capable of generating an unbelievable quantity of RFI (EMF) over a very wide frequency spectrum? Along a similar line - Has a broadband over powerline (BPL) internet access recently been installed?
A loose ground or neutral will cause this problem in computer equipment (and presumably sound equipment), the ole “60 hertz hum”…
Do a visual inspection of the main gounding system for your service. In most instances this will be located outside around the meter can. A #4 gauge solid copper wire is typcally connected between the meter can and one or two ground rods driven at least 6 feet apart. If your grounding system is configured like this then:
Is the ground wire bare or in conduit?
Is the wire corroded where it enters the box or clamps?
Is there one or two ground rods? (Some localities in Florida require 2 ground rods due to poor conductivity in sand.)
If you do suspect a bad ground (or neutral) connection, have a licensed electrician check your ground and tighten all hot and neutral wire lugs in the meter and main panel.
Let us know what you find out…