Strange behavior from a circuit, please explain
In this circuit, there is a negative feedback from the collector to the base, through the resistor R1. Another implicit feedback is possible through the capacitor if the battery is discharged enough, because of the internal resistance:







simulate this circuit &ndash; Schematic created using CircuitLab

When there are inductors, capacitors, amplifier and two "negative" feedbacks, it is always possible for some frequency the negative feedback to become positive and the schematic start to oscillate. 

Try to set a decoupling capacitor as showed in my schematic.

note: R2 in your schematic is actually superfluous and can be safely removed. It does not affect the work of the schematic in any way. Such resistor is needed when current feedback (with an emitter resistor) is used.