I have just bought a Bachman NG7 "Una" Hunslet 040. Great to look at. But as new first use the engine waddles down the track and it makes a rubbing noise so not as quiet as its sister Britomart (also a Hunslet 040). Is there a simple fix or should i return to manufacturer ?
That was a problem with prototype 0-6-0's and other engines without a pony truck upfront. The whole purpose of the leading truck on prototype engines was to minimize or stop the waddling, also called "hunting." That is one big reason why locomotives without pony trucks were not used on mainline trains much. Hunting would develop at track speed and would allow the engine to pick at the most molecular misaligned rail joint and possibly derail. Our models can do much the same.
If you have access to an NMRA standards gauge or something similar, check the wheel gauge on the leading wheel and see if it is slightly narrower than it ought to be. That would permit excessive sideways wheel play and allow the engine to hunt. If it is, there's not a whole lot you can do about it without Advanced tools and skills. You might call the Bachmann Service department and see what they have to say.
One last point to keep in mind, and forgive me if you already know this, but little industrial locomotives like this probably rarely ran more than five or 6 miles an hour in their typically small environment on the company's property. A lot of modelers don't realize this and run 0-4-0's and 0-6-0's at mainline track speed, pulling complete trains, which was rarely ever done on the prototype. A few years ago I read one of the last firsthand steam era crew narratives, those guys are nearly all passed now, from a fireman on a Union Pacific 0-6-0 that had been assigned to a local freight during a World War II power shortage. He described in great detail the hunting that went on when that engine got above about 25 miles an hour, which it had to to maintain a schedule on the main line heavily overloaded with wartime traffic.