Transmission whine that only shows up on the highway, is a specialist worth it?

[primitivoconner]

There's a faint whine coming from my 320d that only really shows up once I'm cruising above 100 and it's been getting slightly louder over the past few weeks. Around town I hear nothing, which is why I keep second guessing myself and wondering if I'm being paranoid. A friend reckons I should look into Quality automatic transmission repair for BMW cars in Shabiya before it turns into something bigger, but I'm torn between that and just letting a general garage have a look since it's cheaper. My worry is that a general garage will shrug and tell me it's normal, whereas a specialist might actually pinpoint it. Has anyone dealt with a noise like this? And is it worth paying more for someone who only works on these cars, or am I overthinking it?