Timing results gone awry

[Gordon Palmer]

I've run REW on a Macbook for some time with UMIK-1 and a Minidsp 2X4 HD. Prepro is Marantz AV7705
Today running a routine sweep of my subs, the timing results are all over the place, into 100's of ms and varying wildly from one sweep to the next.
I'm using acoustic timing to the Left channel.
I've never had this problem in 100's of previous measurements.
I tried sweeps on Left and Right channels to eliminate the Minidsp from the equation and it was the same issue.
I haven't changed any settings.
Suggestions what to try next please?
How would I implement Loopback timing with my setup?

 

[John Mulcahy]

You cannot use a loopback connection with a USB mic. Do you hear the timing reference signal from the left speaker when you make the sub measurements? The timing reference signal has to go to a speaker with a tweeter as it is a high frequency sweep. For further comment it would be best to post an mdat file with some of your measurements.

 

[Gordon Palmer]

Yes I hear the usual chirp.
MDAT attached.
Re running the same sweep multiple times gives different timing and frq plot every time.

 

[John Mulcahy]

The problem is that the measurements are very noisy (about 100 times more noise than would typically be seen), so the detection of the impulse response start time is unreliable. Not sure why that would be, do you hear the sub sweeps OK? Is there a lot of background noise?

You are also using an older beta version, though that is unrelated.

 

[Gordon Palmer]

There was a low level hum..not enough to bother me in normal listening but I rearranged a few power cables at the back of my rack and it has reduced significantly.
Timings have returned to normal so this may have been the cause.
Thanks for help.