AD7770 values contains modulated high power noise

We have AD7770 running at 32 KSps, SPI mode, single-ended. For testing, we feed in pure sine wave (for example 440Hz) into a single channel. Output values contains high pitch noise, after FFT it shows we have 2 strong (and sharp) peaks at 8 KHz and 16 KHz. 

These two peaks are repeated exactly at 1/4 and 1/2 of sampling rate, so if we change sampling rate to 22 Ksps, noise moves to 5.5 KHz and 11 KHz. 
Input signal is generated with stable signal generator, and as well we check the signal with scop to be sure we do not have these unwanted signals at input pins.

Gain and offsets are default, buffer mode disabled, output is SD mode. VRef is 1.2 volt, VRms of signal is 500 mv. 

I do appreciate any help or suggestion.

Keivan