Current noise of the OPA637 op-amp

Hi,

We have a preamplifier design using the OPA637AU and are experiencing high noise levels (~30nV/rtHz, rti) apparently due to current noise.  The input source presents approximately 330pF//10M Ohms to the + input, and when shorted, the noise drops to the expected ~4.5nV/rtHz, rti. Also, the noise corresponds to current through a capacitive load, rising as frequency decreases (70 - 90kHz) indicating an equivalent I_n of 4.7pA/rtHz at 75kHz.  This is way beyond the specified 2.5fA/rtHz called out in the data sheet.  The op-amp is configured for a non-inverting gain of 27dB with Rg=100 Ohms and Rf =2.21k Ohms (no feedback capacitor).  The date (lot?) code on the device is 247QT and I have tried 2 units so far with the same results.

What am I missing?

Thanks for the help

Lyman