image problems

[Hal Coxon]

I'm a novice
I have a 20 d w 100-400 IS zoom - I love it
Taking mainly HS sports pics of my kids
Recently - my images have faint horizontal lines running from side to side.
All of the images were shot indoors.
I haven't had time to touble shoot the problem but I'm assuming with all the experience that visits this forum someone probably has seen this before.
Unfortunately images are too large to send as attachments
Any ideas
Thanks

[Mike Panic]

go outside on a sunny bright day, take the lens off and shoot a pic of the sky

look for anything unusual.

do the same w/ your 100-400 dust pump @ widest apreture, and the same w/ any other lens