Your first post said the power supply noise was 27 - 30 KHz, but your last post said it was 28 MHz.
You may want to post your observations on the E2E forum that supports the PMIC to see if they have any suggestions.
I would like to provide an additional comment related to using a crystal circuit vs LVCMOS oscillator. I do not know any reason you should not use a 1.8 volt LVCMOS oscillator. We just do not see them used very often because customers prefer to purchase the lower cost option which is the crystal. I think you would be seeing the same power supply noise if you were using a crystal circuit rather than the LVCMOS oscillator. Using an LVCMOS oscillator could improve noise immunity of your product if it is going to be exposed to a environment with lots of electrical noise. There have been cases where noise has coupled into the crystal circuit which cause it to be injected on the slow changing sinusoidal signal generated by the crystal circuit and this noise was large enough to generate glitches on the internal clock signal as the noisy sinusoidal signal transitions through the switching threshold of the input buffer. See Advisory 1.0.30 in the AM335x Silicon Errata for more details.
Regards,
Paul