FIR filter artifact phases

Hi! I'm doing a filter on my timecourses, which had a 0.5Hz noise I want to get rid of. I used FIR low pass filter(Parks-McClellan algorizm), and I got rid of the noises. But an extra peaks and troughs were introduced in the data(see the plot figure). I think it's an artifact. Can someone tell me how to improve my filter?

the blue curve is the original data, red one is after lowpass. The artifact I was mentioning is the red curve near x axis 150.

[Image Analyst]

Do you think if we saw your code it might be easier for us to help you? Just a thought....