GLHI - as mentioned elsewhere, the 737 requirement is that you are out of operating reverse (that is up to max) by 60 kts and then at reverse idle by normal taxy speed. Surges are unlikely if you operate the 737 in that way. There is no requirement to be at idle reverse at 60kts.
Mr David - you have not told us where you were when you heard these 'bangs' - were you on board? Could they have been the undercarriage locking down perhaps?