LM386 noise when using 5v power supply
The most likely cause is noise from the power supply.

5V USB power supplies assume that they don't have to clean up the output from the switching power supply, since you are going to use it to charge a device - the device's charging circuit doesn't care about noise, and the device itself runs off of the battery.  

By the time the current from the charger reaches any audio stages, it has been through the charge regulator, various amounts of filtering, and the battery itself (which can swallow some/all of the noise.)

But, you are using it to power the amplifier directly so you get the full effect of the switching noise.

This guy ran some tests.  

As you can see by the oscilloscope pictures, there's a lot of noise.
Like this:


There's better ones and worse ones, but yeah, noise from a USB charger is to be expected.

You'll need to do some filtering of your 5V and GND lines if you want to use that charger.

Easiest is probably getting hold of a 9Volt charger and using a 5Volt linear regulator. You don't need an LDO (Low DropOut) regulator, just a garden variety linear regulator like the 7805 you mentioned.

Actually, you could probably run the amp straight off of the 9V power supply if it is clean.  If not, use your 7805.