Background Noise ( lines ) in the image

[Ashoksomeshwar]

I am using an A 7 iii with 200 -600G. I have just purchased it and I have noticed that in my image if the background is not too far, then there are lines appearing on the background. The run slant upwards from left to right. Though the same can be mitigated to a certain extent by Noise removal, they should not have been there in the first place. I am attaching two images where the same can be seen in the background. The Camera was on Aperture priority mode for the first image at F/6.3, SS 1/1000, ISO 1000 , @ 600mm. Exposure at +0.3.
Any suggestions are appreciated.
Thanks
Ashok

• ILCE-7M3
• FE 200-600mm F5.6-6.3 G OSS
• 600.0 mm
• ƒ/6.3
• 1/1000 sec
• ISO 1000

• ILCE-7M3
• FE 200-600mm F5.6-6.3 G OSS
• 600.0 mm
• ƒ/6.3
• 1/2500 sec
• ISO 400