Bad 100Hz ripple in linear regulator circuit
1) In general you would not use 2 bridge rectifiers to make 2 x rectified 12 VAC (that's 17 V DC peak) and connect those in series. It is more common to connect the transformer windings in series (so you get 24 V AC) and feed that to the bridge rectifier.

2) I hope you're not really using a 1N4148 at the input of the LM317, a 1N4148 is a signal diode not a power diode. It cannot handle more than 100 mA DC and also will drop quite some voltage when you put 100 mA through it. Replace it with a 1N4001 and keep the current below 1 A. For more than 1 A you will need a different diode and also more than 1 A will be pushing the LM317 too hard, then you should consider a switched regulator solution.

3) the 100 Hz ripple might also be related to grounding issues. Do make sure you follow the "star ground" principle, read about that here.