Hallo James,

once upon a time I had a stressing loud cooling fan, for my PC processor. So I decided to make him softer. I inserted a series resistor, and indeed he became very soft. Voltage drop accross series resistor was only about 3V and I thought everything is fine.
Because I am a paranoid person {grin} I had a look at the cooling fan at the next power-on. Everything was ok. Then, I closed enclosure and began to work. To my surprise after the next power-on cooling became really soft. First I was lucky about the success of my modification. But then I realized, that coolong fan was not turning, at all. Having a look at him I saw that he was jerking all the time, like a dying heart. It was terrible...

Well, what I want to tell you, is, that a DC-motor always needs a much higher voltage to reliably start, than for keeping running. Do never apply a voltage to your DC-motor which is not specified by manufacturer. Means, if specification is 6V...15VDC, then you can apply 6V to slow down your motor. But if specification is 12DC only, then you must be very carefully. Have a look at datasheet for minimum voltage, and never apply a voltage smaller than specified.

The only methode for controlling speed of motor laying outside of that what can be realized with using allowed DC voltage range, is PWM.

Let's go on with my little story:

After I realized that I could not make the cooling fan softer with DC voltage decreasing I decided to use PWM. You know 555-methode with some suited transistor. Everything worked fine now. Rotation speed could be slowed down extremely, and starting was very reliably. BUT: Performance was NOT softer. Now click sounds could be heard from fast edges: tack tack tack tack tack tack...
And result was even more stressing. So, the only way to proceed was to buy a good cooling fan...

Back to your problem: I would use PWM, but don't be surprised about increasing noise.
On the other hand, if your motor is very small and current drawn is also small, some series resistor might be appropriate. But have a look with oscilloscope at starting your motor. Voltage accross your motor must not drop below specified minimum DC voltage.

Bye,
Kai