Noisy Seagate Ironwolf 6TB drives

I recently bought my 4th ReadyNAS box (a 524) after having three older models on OS 4.2.30.  I installed Seagate Ironwolf 6TB drives in the 524 but I am getting a lot of noise from the drives - particularly when they spin up from standby and then less so with drive use. I have never experienced any noise like this from my 3 older Readynas units (3TB and smaller drives) or on the two recent Readynas boxes (a 316 and a 524) that I've set up for friends (4 TB Ironwolf drives).  Is this a normal noise for the larger Ironwolf drives or did I get a bad batch of drives ?

Spin up itself generates a lot of noise for about 30 sec and it's scared me off using spin down at all.  The noise seems to go on for awhile and I don't know if it is each drive spining up one by one or whether one particular drive is having trouble starting?  

I have never experienced any noise like this from my older boxes (3TB and smaller drives) or on the recent 316 and 524 boxes I've set up for friends (4 TB Ironwolf drives).  Is this a normal noise for the larger Ironwolf drives or did I get a bad batch ?

Recording from my iphone attached (not great quality but you can hear the noise)