Any FM modulator will have interference at times. thats why you have several station options to chose from. And I have yet to hear an FM Modulator that sounds decent..
Honestly you best option is to replace your stock HU with aftermarket. Even the cheap Dual headunits will sound better, look better, and offer you an aux port. If your going through all the trouble to wire up a FM modulator, drill holes and mount the thing. You could have gone aftermarket and got an all around cleaner better looking, and Much better sounding system.
You can buy a lightning audio HU on crutchfield for $59.99 and $15 for the install gear it has an aux jack and 2 usb ports all for $75! But buy a HU over $119.99 and the install gear is free! Check it out