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usually 100 uF capacitor is used as bypass and filter noise in PSU (5 V to 12 V), as capacitors do not allow DC to pass they are used and i think there is no specific in frequency... where 30 MHz noise is present in your PSU...
I doubt a 100uF capacitor will be effective at 30MHz. I would go for one or more 100nF ceramic capacitors with short wires or even use SMD types if possible. If you have to block the noise enteringor leaving part of the supply you may have to use an inductor as well as capacitors. What is creating the 30MHz and where is it causing a problem?
As betwixt said, 100 nF is a good choice for this frequency. Attached is a plot of impedance vs frequency for a 100 nF, 0603, X5R, 16 V cap. This is from Murata's Simsurfing tool. It has minimum impedance at around 20 MHz and will definitely work well for noise at 30 MHz.
The package size will have a large affect on the resonant point (minimum impedance), if you need higher frequency go with smaller package, lower frequency - larger package.