Trimming fastq up until a sequence

Hi

I have a somewhat difficult problem to find a solution to on google. I need to trim my fastq files up until a sequence, and not remove that sequence (but remove everything before it).

This 
someRandomNoise_aKnownSequence_unknownSequence 
becomes
aKnownSequence_unknownSequence

All the tools I use, and that I have seen, would remove both the "someRandomNoise" and the "aKnownSequence"

I could try to find the location of the sequence in each read and then trim then in a loop, but this seem very inefficient.