It won't make the coil totally ineffective but it might make it less effective.
It absolutely will not attenuate the signal. It's just some coax, doesn't matter how you coil it. The signal picked up by the antenna that makes its way to the radio is ONLY ON THE INSIDE .
It is *possible* that the coil could make the antenna *more* susceptible to noise but it's very unlikely.
The way it could happen is described in here:http://www.audiosystemsgroup.com/RFI-Ham.pdf
Search for "threshold impedance"
It is possible to add a choke to a feedline and accidentally resonate some undesired path from antenna to ground but I doubt you did that.
The file above is a good read. It's pretty technical but it will help you a lot in your attic antenna situation if you keep it handy.